sunxi: Makefile: Drop explicit targets built by binman

On sunxi various files that need to be created by binman. It does not make
sense to enumerate these in the Makefile. They are described in the
configuration (devicetree) for each board and we can simply run binman
(always) to generate them.

This avoid sprinkling the Makefile with arch-specific code.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Simon Glass <sjg@chromium.org>