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>

board/sunxi/README.sunxi64

@@ -38,6 +38,12 @@ the root of your U-Boot build directory (or create a symbolic link).
 $ export BL31=/src/arm-trusted-firmware/build/sun50iw1p1/debug/bl31.bin
   (adjust the actual path accordingly)
 
+If you run into size issues with the resulting U-Boot image file, it might
+help to use a release build, by using "DEBUG=0" when building bl31.bin.
+As sometimes the ATF build process is a bit picky about the toolchain used,
+or if you can't be bothered with building ATF, there are known working
+binaries in the firmware repository[3], purely for convenience reasons.
+
  SPL/U-Boot
 ------------
 Both U-Boot proper and the SPL are using the 64-bit mode. As the boot ROM