powerpc/spider-pic: Get pic from chip_data instead of irq_map

Building on Grant's efforts to remove the irq_map array, this patch
moves spider-pics use of virq_to_host() to use irq_data_get_chip_data
and sets the irq chip data in the map call, like most other interrupt
controllers in powerpc.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>