MIPS: Octeon: Fix warning in of_device_alloc on cn3xxx

Starting with commit 3da5278727a895d49a601f67fd49dffa0b80f9a5 (of/irq:
Rework of_irq_count()) the following warning is triggered on octeon
cn3xxx:

[    0.887281] WARNING: CPU: 0 PID: 1 at drivers/of/platform.c:171 of_device_alloc+0x228/0x230()
[    0.895642] Modules linked in:
[    0.898689] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.14.0-rc7-00012-g9ae51f2-dirty #41
[    0.906860] Stack : c8b439581166d96e ffffffff816b0000 0000000040808000 ffffffff81185ddc
[    0.906860] 	  0000000000000000 0000000000000000 0000000000000000 000000000000000b
[    0.906860] 	  000000000000000a 000000000000000a 0000000000000000 0000000000000000
[    0.906860] 	  ffffffff81740000 ffffffff81720000 ffffffff81615900 ffffffff816b0177
[    0.906860] 	  ffffffff81727d10 800000041f868fb0 0000000000000001 0000000000000000
[    0.906860] 	  0000000000000000 0000000000000038 0000000000000001 ffffffff81568484
[    0.906860] 	  800000041f86faa8 ffffffff81145ddc 0000000000000000 ffffffff811873f4
[    0.906860] 	  800000041f868b88 800000041f86f9c0 0000000000000000 ffffffff81569c9c
[    0.906860] 	  0000000000000000 0000000000000000 0000000000000000 0000000000000000
[    0.906860] 	  0000000000000000 ffffffff811205e0 0000000000000000 0000000000000000
[    0.906860] 	  ...
[    0.971695] Call Trace:
[    0.974139] [<ffffffff811205e0>] show_stack+0x68/0x80
[    0.979183] [<ffffffff81569c9c>] dump_stack+0x8c/0xe0
[    0.984196] [<ffffffff81145efc>] warn_slowpath_common+0x84/0xb8
[    0.990110] [<ffffffff81436888>] of_device_alloc+0x228/0x230
[    0.995726] [<ffffffff814368d8>] of_platform_device_create_pdata+0x48/0xd0
[    1.002593] [<ffffffff81436a94>] of_platform_bus_create+0x134/0x1e8
[    1.008837] [<ffffffff81436af8>] of_platform_bus_create+0x198/0x1e8
[    1.015064] [<ffffffff81436cc4>] of_platform_bus_probe+0xa4/0x100
[    1.021149] [<ffffffff81100570>] do_one_initcall+0xd8/0x128
[    1.026701] [<ffffffff816e2a10>] kernel_init_freeable+0x144/0x210
[    1.032753] [<ffffffff81564bc4>] kernel_init+0x14/0x110
[    1.037973] [<ffffffff8111bb44>] ret_from_kernel_thread+0x14/0x1c

With this commit the kernel starts mapping the interrupts listed for
gpio-controller node. irq_domain_ops for CIU (octeon_irq_ciu_map and
octeon_irq_ciu_xlat) refuse to handle the GPIO lines (returning -EINVAL)
and this is causing above warning in of_device_alloc().

Modify irq_domain_ops for CIU and CIU2 to "gracefully handle" GPIO
lines (neither return error code nor call octeon_irq_set_ciu_mapping
for it). This should avoid the warning.

(As before the real setup for GPIO lines will happen using
irq_domain_ops of gpio-controller.)

This patch is based on Wei's patch v2 (see
http://marc.info/?l=linux-mips&m=139511814813247).

Signed-off-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Reported-by: Yang Wei <wei.yang@windriver.com>
Acked-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6624/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>