pinctrl: realtek: Select REGMAP_MMIO for RTD driver

The pinctrl-rtd driver uses 'devm_regmap_init_mmio', which requires
'REGMAP_MMIO' to be enabled.

Without this selection, the build fails with an undefined reference:
aarch64-none-linux-gnu-ld: drivers/pinctrl/realtek/pinctrl-rtd.o: in
function rtd_pinctrl_probe': pinctrl-rtd.c:(.text+0x5a0): undefined
reference to __devm_regmap_init_mmio_clk'

Fix this by selecting 'REGMAP_MMIO' in the Kconfig.

Fixes: e99ce78030db ("pinctrl: realtek: Add common pinctrl driver for Realtek DHC RTD SoCs")
Signed-off-by: Yu-Chun Lin <eleanor.lin@realtek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>