MIPS: boot: do not include $(cpp_flags) for preprocessing ITS

$(CPP) is used here to perform macro replacement in ITS.  Do not
pass $(cpp_flags) because it pulls in more options for dependency
file generation etc. but none of which is necessary here.  ITS files
do not include any header file, so $(call if_change,...) is enough.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Patchwork: https://patchwork.linux-mips.org/patch/19093/
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org