Add debounce support for ep93xx gpio interrupts.

On the EP93xx, GPIO ports A, B, and F can be used to generate
interrupts. For each port, if interrupts are enabled, it is
possible to debouce the input signal. Debouncing is implemented
by passing the input signal through a 2-bit shift register clocked
by a 128Hz clock.

This patch adds a platform specific way to enable the debouce
feature for these input ports.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>