drivers: pinctrl: gd32: initial support for AFIO based SoCs

Add a pin control driver for GD32 SoCs using the AFIO model.

Thanks to Gerson Fernando Budke for testing and implementation
suggestions.

Co-authored-by: Gerson Fernando Budke <gerson.budke@atl-electronics.com>
Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>