malta: Use device model & tree for UART

Make use of device model & device tree to probe the UART driver. This is
the initial step in bringing Malta up to date with driver model, and
allows for cleaner handling of the different I/O addresses for different
system controllers by specifying the ISA bus address instead of a
translated memory address.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>