[ARM] gic: Export irq chip functions

[ARM] gic: Export irq chip functions

Some systems combine the GIC with an external interrupt controller.
On these systems it may be necessary to update both the GIC registers
and the external controller's registers to control IRQ behavior.
Export the irq chip functions so that these systems can define a
custom irq chip that calls into the GIC handlers.

Change-Id: I17fc4440fa2c91cc63004abf69c8e872b55c83c6
Signed-off-by: Colin Cross <ccross@android.com>