plat: pass -D option to BL*_CPPFLAGS instead of BL*_CFLAGS

-D is a preprocessor flag that defines a macro. So, adding it to
BL*_CPPFLAGS makes more sense. You can reference it not only from
.c files but also from .S files.

Change-Id: Ib4f2f27a3ed3eae476a6a32da7ab5225ad0649de
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>