gpio: zynq: protect direction in/out with a spinlock

Fix race condition when changing the direction (in/out) of the GPIO pin.
The read-modify-write sequence (as coded in the driver) isn't atomic and
requires synchronization (spinlock).

Signed-off-by: Glenn Langedock <Glenn.Langedock@barco.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>