ArmPlatformPkg: PL011 Dynamic clock freq Support

ArmPlatformPkg: PL011 Dynamic clock freq Support

Some platform support dynamic clocking, which is controlled by some
jumper setting or hardware registers. Result of that is that PCD
PL011UartClkInHz would need to be updated for frequency change.

This patch implements support for dynamic frequency for PL011 uart.

This patch implements default lib, which is using Pcd. Platform which
needs dynamic clocking needs implement PL011UartClockLib

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Udit Kumar <udit.kumar@nxp.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>