OMAP3: GPIO: Only enable WAKEUPEN for edge detection GPIOs

According to the GPIO 'Wakeup and Interrupt' section of the TRM[1],
wake-up requests can only be generated on edge transitions.

Also for OMAP3, only edge GPIOs may lose interrupts when PER enters
RET/OFF state, this is addressed by gpio prepare|resume idle functions

[1] Section 25.5.3.1 OMAP34xx_ES3.1_TRM_V_Q

Signed-off-by: Chunqiu Wang <cqwang@motorola.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>