arm64: dts: allwinner: h6: orangepi-3: Add eMMC node

OrangePi 3 can optionally have 8 GiB eMMC (soldered on board). Because
those pins are dedicated to eMMC exclusively, node can be added for both
variants (with and without eMMC). Kernel will then scan bus for presence
of eMMC and act accordingly.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>