mmc: dt: usdhi6rol0: add optional pinctrl binding

Add a pinctrl binding to specify different pin settings for high speed
modes and UHS modes.

Signed-off-by: Lars Persson <larper@axis.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>