arm64: allwinner: a64: add DTSI file for SoPine SoM

SoPine is a SoM by Pine64, which have a gold finger compatible with the
slot of DDR3 SODIMM (signals are not compatible), and have an A64, an
AXP803, a LPDDR3 DRAM chip, a power led and a MicroSD slot on it.

The card detect pin of the MicroSD slot on the SoM is pulled down, which
makes it unusable; however, the slot is at the surface of the SoM that
is closed to the baseboard, so it's nearly impossible to hot-swap it,
thus I make it non-removable.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>