Silicon/Bcm283x: Add UART constants for PL011 and miniUART

This adds offset, base address, interrupt and register-space length
for the 2 UARTs that the Bcm283x SoC provides.

To be consistent, we simplify the two other existing base address
definitions to a more legible equivalent since there is no point
in explicit refs to FixedPcdGet64 (PcdBcm283xRegistersAddress).

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>