pinctrl: sunxi: Fix potential null pointer dereference

kzalloc can fail. Add a null check to avoid null pointer
dereference error while accessing the pointer later.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>