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 <marek.vasut+renesas@gmail.com>
Cc: Alex Kiernan <alex.kiernan@gmail.com>
Cc: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Cc: Eugeniu Rosca <roscaeugeniu@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick DELAUNAY <patrick.delaunay@st.com>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cc: Simon Glass <sjg@chromium.org>