mmc: usdhi6rol0: add pinctrl to set pin drive strength

Some boards need different pin drive strength for the UHS mode. Add an
optional pinctrl setting with two pin states covering UHS speeds and
other speeds.

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