Makefile: sunxi: Don't use binman to build ATF image

At present with sunxi 64-bit, the Makefile builds
u-boot-sunxi-with-spl.bin and then binman overwrites it with its own
version. But the binman definition lacks some parts, in particular
BL31.

For now, work around this with a hack.

Tested-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 42b18df8 ("x86: Makefile: Drop explicit targets built by binman")