Platform/FVP-AArch64: use different serial ports for DEBUG and console

The FVP models expose several emulated serial ports, and always start with
Xterm windows connected to at least two of them. So let's switch to the
second one for DEBUG output, leaving the original one for console output
via SerialDxe.

Note that explicit SerialPortWrite() calls made from other modules will
now also end up on the DEBUG console. Conversely, DEBUG output generated
in the context of SerialDxe will end up on the primary serial console.
This is a fundamental consequence of the way DEBUG() that is built on top
of SerialPortWrite(), which can only be implemented in one way in each
module (but can differe between modules, hence this patch)

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>