pinctrl: nomadik: fix incorrect type in return expression

Sparse reports "warning: incorrect type in return expression (different
address spaces)" because nmk_gpio_populate_chip() is supposed to return
(struct nmk_gpio_chip *) whereas devm_ioremap_resource() returns
(void __iomem *).  ERR_CAST() is needed to fix the warning.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>