gpio/omap: remove omap_gpio_save_context overhead

Context is now saved dynamically in respective functions whenever and
whichever registers are modified. This avoid overhead of saving all
registers context in the runtime suspend callback.

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>