sunxi: update Pine64 README

With the DRAM init code and the SPL's ability to load the ATF binary as
well, we can now officially get rid of the boot0 boot method, which
involed a closed-source proprietary blob to be used.
Rework the Pine64 README file to document how to build the firmware.
Also since these instructions now cover more boards, rename the
file to README.sunxi64 to reflect this.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>