serial: ralink: adds support for the serial core found on ralink wisoc

The MIPS based Ralink WiSoC platform has 1 or more 8250 compatible serial cores.
To make them work we require the same quirks that are used by AU1x00.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>