pinctrl: gpio: Add callback for configuring pin as GPIO

pinctrl: gpio: Add callback for configuring pin as GPIO

Add callback to configure, and de-configure, pin as a GPIO on the
pin controller side. This matches similar functionality in Linux
and aims to replace the ad-hoc implementations present in U-Boot.

Signed-off-by: Marek Vasut <[email protected]>
Cc: Alex Kiernan <[email protected]>
Cc: Christoph Muellner <[email protected]>
Cc: Eugeniu Rosca <[email protected]>
Cc: Patrice Chotard <[email protected]>
Cc: Patrick DELAUNAY <[email protected]>
Cc: Philipp Tomsich <[email protected]>
Cc: Simon Glass <[email protected]>