arm64: dts: zte: add mmc devices for zx296718

Add three mmc devices for zx296718 SoC, and enable the SD and eMMMC on
zx296718-evb board.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>