[POWERPC] Xilinx: boot 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 adressing. With
additional properties it is compatible with the open firmware
'ns16550' compatible binding.

This code updates the ns16550 driver to use the reg-offset property
so that the Xilinx UART 16550 can be used with it. The reg-shift
was already being handled.

Signed-off-by: John Linn <john.linn@xilinx.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>