MAINTAINERS: add include/dt-bindings/pinctrl to PIN CONTROL SUBSYSTEM

MAINTAINERS: add include/dt-bindings/pinctrl to PIN CONTROL SUBSYSTEM

Maintainers of the directory Documentation/devicetree/bindings/pinctrl
are also the maintainers of the corresponding directory
include/dt-bindings/pinctrl.

Add the file entry for include/dt-bindings/pinctrl to the appropriate
section in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20220613122955.20714-1-lukas.bulwahn@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>