Instead of open coding logic for reading and writing GPIO lines,
use the generic GPIO library. Also switch to using the spinlock
from the generic GPIO to protect the registers.

Cc: linux-snps-arc@lists.infradead.org
Acked-by: Christian Ruppert <christian.ruppert@alitech.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>