ARM: mach-shmobile: sh73a0 GPIO IRQ support

This patch adds support for sh73a0 GPIO IRQs by making use
of the PFC GPIO IRQ feature. Only IRQ pins are  supported
at this time. In the future when PINT interrupts also are
supported properly we can easily extend the table with such
information. Also, the sh73a0 is currently making use of
the GIC for external interrupt which is rather unflexible
when it comes to triggering configuration at this point.

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