arm64: dts: meson-g12: split emmc pins to select 4 or 8 bus width

The Khadas VIM3 shares the eMMC pins 4 to 7 with the SPI NOR, in order
to enable the eMMC and the SPI NOR interface, we need to omit the
4 last pins from the eMMC pinctrl.

As it was done for the Khadas VIM2, split the eMMC pinctrls in ctrl, data
and ds pins with either 4bits data or 8bits data, and update the current
board accordingly.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200313090713.15147-2-narmstrong@baylibre.com