ArmVirtPkg/FdtPL011SerialPortLib: Set the PL011 UART clock rate

ArmVirtPkg/FdtPL011SerialPortLib: Set the PL011 UART clock rate

The interface to PL011UartInitializePort has changed in
ArmPlatformPkg/Drivers/PL011Uart with the title:
"ArmPlatformPkg: Add support to configure PL011 UART clock"

This patch updates the calls to PL011UartInitializePort(), in line with
that change, adding a parameter value using the PCD previously used
directly by the driver.

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>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>