[ARM] 4670/1: ep93xx: implement IRQT_BOTHEDGE gpio irq sense type

Having a both-edge sensitive irq trigger type is required for the
generic gpio-keys input driver; alas the ep93xx does not support
both-edge gpio triggers in hardware, so this patch implements them by
switching edge polarity on each triggered interrupt.  This is the same
approach as taken by the Orion SoC both-edge gpio irq support
implementation.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>