powerpc: remove redundant header search path additions

The same path -Iarch/$(ARCH) is passed to KBUILD_CPPFLAGS,
KBUILD_AFLAGS, and KBUILD_CFLAGS.

As you see in scripts/Makefile.lib, KBUILD_CPPFLAGS is passed
to c_flags and a_flags as well.

Passing it to KBUILD_CPPFLAGS is enough.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>