SC26XX: New serial driver for SC2681 uarts

New serial driver for SC2681/SC2691 uarts.  Older SNI RM400 machines are
using these chips for onboard serial ports.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Torben Mathiasen <device@lanana.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>