ArmPlatformPkg/PL011SerialPortLib: use untyped PCD for register base

ArmPlatformPkg/PL011SerialPortLib: use untyped PCD for register base

Use an untyped PCD reference for PcdSerialRegisterBase, so that the
library gets built without hardcoded values, permitting modules to
override the default serial port. This allows SerialDxe to use a
different serial port from the one used for DEBUG output (which
often gets occluded due to the console driver clearing the screen).

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>