openrisc: add arch/openrisc/Kbuild

Describe the subdirectories under arch/openrisc/ in arch/openrisc/Kbuild
so you can use the standard obj-y syntax.

I removed the CONFIG_OPENRISC_BUILTIN_DTB conditional because it is
already controlled by arch/openrisc/boot/dts/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>