[POWERPC] Xilinx: of_serial support for Xilinx uart 16550.

The Xilinx 16550 uart core is not a standard 16550 because it uses
word-based addressing rather than byte-based addressing. With
additional properties it is compatible with the open firmware
'ns16550' compatible binding.

This code updates the of_serial driver to handle the reg-offset
and reg-shift properties to enable this core to be used.

Signed-off-by: John Linn <john.linn@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>