gpio/omap: save and restore debounce registers

GPIO debounce registers need to be saved and restored for proper functioning
of driver.

Signed-off-by: Nishanth Menon <nm@ti.com>
tarun.kanti@ti.com: Debounce context save is moved to _set_gpio_debounce()
as part of dynamic context save to remove overhead.
Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>