dm: ns16550: Add support for reg-offset property

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 <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Tom Rini <[email protected]>