[ARM] 3374/1: ep93xx: gpio interrupt support

Patch from Lennert Buytenhek

Add GPIO interrupt support for the first 16 GPIO lines (port A
and B.)

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>