pinctrl: nuvoton: wpcm450: Fix build error without OF

If OF is not set, bulding fails:

drivers/pinctrl/nuvoton/pinctrl-wpcm450.o: In function `wpcm450_dt_node_to_map':
pinctrl-wpcm450.c:(.text+0x404): undefined reference to `pinconf_generic_dt_node_to_map'

Make PINCTRL_WPCM450 depends on OF to fix this.

Fixes: a1d1e0e3

 ("pinctrl: nuvoton: Add driver for WPCM450")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Link: https://lore.kernel.org/r/20220325074450.3228840-1-zhengbin13@huawei.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>