Platform/RPi4: Add switch to select between PL011 and miniUART

The PL011 can be a better choice for the serial console on the RPi4,
given that its baud clock is not derived from the CPU clock (which
may change under our feet unless we keep it fixed at a low rate), and
given the fact that the SBSA/SBBR specs that describe ARM specific
architectural requirements for ACPI only permit PL011 based UARTs to
begin with.

Therefore we add a new PL011_ENABLE build switch to tell the firmware
to use PL011 for all console serial I/O, including for TF-A, SPCR and
DBG2, as well as toggle the BlueTooth module to use the mini UART.

For the time being, the option is disabled by default because it
requires an overlay to be enabled in config.txt that pinmuxes the
PL011 TX/RX lines to the UART pins on the connector block.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>