gpio/omap: Use devm_ API and add request_mem_region

Replace the regular kzalloc and ioremap with the devm_ equivalent
to simplify error handling.

Add the missing devm_request_mem_region to reserve the region used
by the driver.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Tarun Kanti DebBarma <tarun.kanti@ti.com>