Add a common routine for parsing FDT data describing an ARM GIC interrupt.

In the fdt data we've written for ourselves, the interrupt properties
for GIC interrupts have just been a bare interrupt number.  In standard
data that conforms to the published bindings, GIC interrupt properties
contain 3-tuples that describe the interrupt as shared vs private, the
interrupt number within the shared/private address space, and configuration
info such as level vs edge triggered.

The new gic_decode_fdt() function parses both types of data, based on the
#interrupt-cells property.  Previously, each platform implemented a decode
routine and put a pointer to it into fdt_pic_table.  Now they can just
list this function in their table instead if they use arm/gic.c.