sh: pfc: Add GPIO IRQ support

Add GPIO IRQ support to the shared PFC code in drivers/sh/pfc.c

The enums pointed out by a certain GPIO will be matched against
a table for IRQ to enum mappings.

Only the shared PFC code is updated by this patch. SoC specific
changes are also needed to allow platforms to make use of this
feature.

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