MIPS: SNI: Switch RM400 serial to SCCNXP driver

The new SCCNXP driver supports the SC2681 chips used in RM400 machines.
We now use the new driver instead of the old SC26xx driver.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4417/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>