malta: Tidy up UART address selection

The address of the UART differs based upon the system controller because
it's actually within the I/O port region, which is in a different
location for each system controller. Rather than handling this as 2
UARTs with the correct one selected at runtime, use I/O port accessors
for the UART such that access to it gets translated into the I/O port
region automatically.

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