sunxi: README.sunxi64: Add hint about non-debug of ARM Trusted Firmware

As we are running into issues where the final U-Boot FIT image file is
exceeding our size limit, add a hint to the README.sunxi64 file
to point out the possibility of building non-debug versions of the ATF
binary. These are about 12KB smaller than the standard debug build, and
so allow successful U-Boot builds for many boards with the Allwinner H5
SoC.
Please note that under normal circumstances the debug build is still
recommended, as it gives valuable clues in case something goes wrong in
the ATF.

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