pinctrl: ns2: Fix potential NULL dereference

platform_get_resource() may fail and return NULL, so we should
better check it's return value to avoid a NULL pointer dereference
a bit later in the code.

Signed-off-by: Young Xiao <92siuyang@gmail.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>