mxc: emulate GPIO interrupt on both-edges

MXC GPIO controller does not support generation of interrupts on both
edges. Emulate this mode in software by reconfiguring the irq trigger
polarity on each interrupt. This follows an example of
drivers/mfd/asic3.c.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>