sunxi: Convert 64-bit boards to use binman

At present 64-bit sunxi boards use the Makefile to create a FIT, using
USE_SPL_FIT_GENERATOR. This is deprecated.

Update sunxi to use binman instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>