serial: remove m32r_sio driver

The m32r architecture is getting removed, so we don't need this
any more.

Acked-by: Nicolas Pitre <nico@fluxnic.net>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>