ARM64: dts: meson-gxl: Take eMMC data strobe out of eMMC pins

Since the Data Strobe pin is optional, take it out of the default
eMMC pins and add a separate entry.

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