ARM64: dts: meson: add mmc clk gate pins

Add the pinctrl to switch mmc clk pins in gpio (pulled down) mode. This
is necessary to be able to gate the clk outside of the SoC while
keeping it running in the controller

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>