kbuild: Descend into SOC directory from CPU directory

kbuild: Descend into SOC directory from CPU directory

Some CPUs of some architectures have SOC directories.
At present, the build system directly descends into SOC directories
from the top Makefile, but it should generally descend into each
directory from its parent directory.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>