dt-bindings: pinctrl: Add dt-bindings for Sunplus SP7021

Add dt-bindings header files and documentation for Sunplus SP7021 SoC.

Signed-off-by: Wells Lu <wellslutw@gmail.com>
Link: https://lore.kernel.org/r/1642344734-27229-2-git-send-email-wellslutw@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>