arm64: dts: allwinner: a64: Add Pine64-LTS device tree file

The Pine64-LTS is a variant of the Pine64 board, from the software
visible side resembling a SoPine module on a baseboard, though the
board has the SoC and DRAM integrated on one PCB.
Due to this it basically shares the DT with the SoPine baseboard, which
we mimic in our DT by inclucing the boardboard .dts into the new file,
just overwriting the model name.
Having a separate .dts for this seems useful, since we don't know yet if
there are subtle differences between the two. Also the SoC on the LTS
board is technically an "R18" instead of the original "A64", although as
far as we know this is just a relabelled version of the original SoC.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>