ArmPlatformPkg: Set Juno debug serial port defaults

ArmPlatformPkg: Set Juno debug serial port defaults

Juno has several serial ports, one of which may be used for a remote
debug interface (e.g. gdb, WinDbg).  The debug serial port needs to
be distinct from that used for UEFI trace to prevent corruption of
debugger protocol messaging.
The UEFI spec requires that serial devices be initialised to default
settings. (11.8 Serial I/O Protocol - "The default attributes for all
UART-style serial device interfaces are: 115,200 baud, ..."
and 17.3.3 EFI Debugport Variable - "These defaults must be used in the
absence of a DEBUGPORT variable...")

This change adds initialization of the serial device reported in the
ACPI DBG2 table.  The initialisation is done early in the boot to allow
the possibility of remote debug of UEFI itself.

NOTE: This is functionally dependent on the DBG2 table being updated in
OpenPlatformPkg, but is required as a precursor to that change.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>