dm: ns16550: Use an address instead of a pointer for the uart base

It is inconvenient to have to use casts when specifying platform data. Also
it is not strictly correct, since we should use map_sysmem() to convert an
address to a pointer.

Adjust the platform data to use an address.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>