On some platforms the UART might have already been initialised, for
instance by firmware running before TF-A or by a separate management
processor. In this case it would not be need to initialise it again
(doing so could create spurious characters). But more importantly this
saves us from knowing the right baudrate and the right base clock rate
for the UART. This can lead to more robust and versatile firmware builds.

Allow to skip the 16550 UART initialisation and baud rate divisor
programming, by interpreting an input clock rate of "0" to signify this
case. This will just skip the call to console_16550_core_init, but still
will register the console properly.

Users should just pass 0 as the second parameter, the baudrate (third
parameter) will then be ignored as well.

Fix copy & paste typos in comments for the console_16550_register()
function on the way.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I9f8fca5b358f878fac0f31dc411358fd160786ee