mediatek: Makefile: Drop explicit targets built by binman

On mediatek 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.

Also update the binman definition so that idbloader.img is only needed
when SPL is actually being used.

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