sunxi: binman: Use a macro for the BL31 load address

This consolidates the SoC-specific part at the top of the file to avoid
cluttering it up with preprocessor conditions.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>