serial: Add 16850 uart type support to OF uart driver

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>