serial: remove cris/etrax uart drivers

The cris architecture is getting removed, so we don't need the
uart driver any more.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>