arm/imx/gpio: add spinlock protection

The GPIO registers need protection from concurrent access for operations that
are not atomic.

Cc: stable@kernel.org
Cc: Juergen Beisert <j.beisert@pengutronix.de>
Cc: Daniel Mack <daniel@caiaq.de>
Reported-by:  <rpkamiak@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>