rpi3: Allow runtime determination of UART base clock rate

rpi3: Allow runtime determination of UART base clock rate

At the moment the UART input clock rate is hard coded at compile time.
This works as long as the GPU firmware always sets up the same rate,
which does not seem to be true for the Raspberry Pi 4.

In preparation for being able to change this at runtime, add a base
clock parameter to the console setup function. This is still hardcoded
for the Raspberry Pi 3.

Change-Id: I398bc2f1e9b46f7af9a84cb0b33cbe8e78f2d900
Signed-off-by: Andre Przywara <andre.przywara@arm.com>