Makefile: fix annoying sunxi hack message

every compilation shows this error

 Hack for sunxi which doesn't have a proper binman definition for
 64-bit boards

not only for sunxi-boards/arm64

fix this by changing to real comments

Fixes: 9f55ee259d0c ("Makefile: sunxi: Don't use binman to build ATF image")
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>