sunxi: A64: Add Pine64-LTS board

The Pine64 LTS is an updated version of the Pine64, copying the
technical updates from the SoPine platform: LPDDR3 DRAM, eMMC socket and
soldered SPI flash chip, even the broken SD card detect pin has been copied.
Consequently this leads to the .dts (copied from the kernel) just including
the SoPine baseboard .dts, and the defconfig being almost identical.
Nevertheless the boards deserves a separate config.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>