pinctrl: add generic board-level pinctrl driver using mux framework

pinctrl: add generic board-level pinctrl driver using mux framework

Many boards use on-board mux chips (often controlled by GPIOs from an I2C
expander) to switch shared signals between peripherals.

Add a generic pinctrl driver built on top of the mux framework to
centralize mux handling and avoid probe ordering issues. Keep board-level
routing out of individual drivers and supports boot-time only mux
selection.

Ensure correct probe ordering, especially when the GPIO expander is probed
later.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>