This patch adds support for "ns16850" as supported value
of the compatible node in flat device tree uart descriptions.

This is needed for example when you have a XR16C2850 uart
connected to a PPC405's external bus controller.
Signed-off-by: Matthias Fuchs <mfuchs@ma-fu.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>