tty: of_serial: add support for the DesignWare 8250

Support the DesignWare 8250 by a new compatible string and registering
the DesignWare helpers.  If the registration of the helpers fails, then
continue as a normal 8250 as we may still get some useful debug out.

Cc: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>