ArmPlatformPkg: Update PL011 Serial PCDs to Fixed PCDs

ArmPlatformPkg: Update PL011 Serial PCDs to Fixed PCDs

The PCDs used in the PL011 UART Driver and Serial Port Library are
inherently "fixed at build".  This change updates the source to use
Fixed PCDs for these values.  This improves clarity and efficiency.

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: Ryan Harkin <ryan.harkin@linaro.org>