ARM: dts: sun8i-h3: Add mmc2 node to the X2

The Beelink X2 has an on-board eMMC so add a node enabling
the mmc2 controller.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>