soc: arm: gigadevice: gd32f4xx: select AF pinmux
soc: arm: gigadevice: gd32f4xx: select AF pinmux

GD32F4XX series have AF based pinmux.

Signed-off-by: Gerard Marull-Paretas <76c1da08be458687d0066397d08e4f7ea9894377@teslabs.com>

galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr