OMAP3: PM: Enable wakeup from ads7846 touchscreen

This  patch enables the wakeup capabilities of ads7846 touchscreen driver.
ads7846 driver can now wakeup the system from suspend on OMAP3430 EVM
and SDP boards.

The earlier approach of enabling wakeup on the touchscreen GPIO pin during
board level mux init is removed. Instead the wakeup flag in
ads7846_platform_data is enabled. Based on the flag, the ads7846 driver
will do an enable_irq_wake which will eventually call into the OMAP GPIO
layer and will enable the wakeup capability on the GPIO pin.

Signed-off-by: Ranjith Lohithakshan <ranjithl@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>