xtensa: build platform directories unconditionally

All of arch/xtensa/platforms/*/ have Makefile. You do not need to
check the presence of Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210811163731.186125-3-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>