frv: Use OFFSET macro in DEF_*REG()

Avoid code duplication by using OFFSET() in DEF_*REG() instead of
replicating the macro.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>