pinctrl: nomadik: Prevent NULL dereference if of_match_device returns NULL

of_match_device() may return NULL.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>