dm: ns16550: Add support for reg-offset property

reg-offset is the part of standard 8250 binding in the kernel.
It is shifting start of address space by reg-offset.
On Xilinx platform this offset is typically 0x1000.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>