ARM: dts: meson8b: ec100: enable the SDHC controller

EC-100 has built-in eMMC flash which is hard-wired to 3.3V VCC (which
means it's limited to high-speed MMC modes). Enable the SDHC controller
to access the contents of the eMMC flash.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200620163654.37207-3-martin.blumenstingl@googlemail.com