Input: ads7846 - add wakeup support

Add wakeup support to the ads7846 driver. Platforms can enable wakeup
capability by setting the wakeup flag in ads7846_platform_data. With this
patch the ads7846 driver can be used to wake the system from suspend.

Signed-off-by: Ranjith Lohithakshan <ranjithl@ti.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>