sunxi: update README.sunxi64

Now that the Allwinner port in the official mainline ARM Trusted
Firmware repository has reached feature parity with the "legacy" ATF
port, let's use the opportunity to update the Allwinner 64-bit build
instructions. This changes:
- Update ATF build instructions to use the mainline repo.
- Add quick command lines for TL;DR people.
- Mention Allwinner H6 build target.
- Mention pre-built FEL binaries.

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