SERIAL: Lantiq: Add driver for MIPS Lantiq SOCs.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Ralph Hempel <ralph.hempel@lantiq.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: alan@lxorguk.ukuu.org.uk
Cc: linux-mips@linux-mips.org
Cc: linux-serial@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2269/
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>