gpio: mxs: Use set and clear capabilities of the gpio controller

The current driver doesn't use the set and clear registers found on the
mxs gpio controller.

This leads the generic gpio controller to be using some internal value
to avoid looking up the value stored in the registers, making it behave
pretty much like a cache.

This raises some coherency problem when a gpio is not modified by the
gpio controller, while it can easily be fixed by using the set and clear
registers.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>