doc: ARMv8: add README.pine64

Since we lack information about the DRAM initialization for the
Allwinner A64 SoC, booting any A64 based board like the Pine64 is a bit
involved at the moment.
Add a README file to explain the process.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Move to board/sunxi/ from doc/]
Signed-off-by: Tom Rini <trini@konsulko.com>