arm: dts: mt8135: Move pinfunc to include/dt-bindings/pinctrl

Move mt8135-pinfunc.h into include/dt-bindings/pinctrl so that we can
include it in yaml examples.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Link: https://lore.kernel.org/r/20210804044033.3047296-1-hsinyi@chromium.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>