ARM: mach-shmobile: Run-time IRQ handler for GIC

Break-out GIC specific IRQ demux code from the file
entry-macro-intc.S and register during run-time.

Covers sh73a0.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>