mfd: Make twl4030 SIH SPARSE_IRQ capable

twl4030 is using a two level irq controllers infrastruture.
So far, only the first level was using dynamic irq_desc allocation
to be able to have irq_domain support for device tree.
There is a need to allocate separate irq_descs for the SIH too to
avoid hacking the first level with interrupts from the second level.

Add an irq_base parameter to allow the caller to provide the base from
pdata or from dynamic allocation.

Affect TWL4030_NR_IRQS to the twl-core IRQs only.

Moreover that will allow the extraction of the of_node pointer for further
Device Tree conversion.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>