pinctrl: at91-pio4: Add COMPILE_TEST support

Add COMPILE_TEST support to the AT91 PIO4 pinctrl driver for better compile
testing coverage.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/1590234326-2194-2-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>