ArmPlatformPkg: Remove UINTN cast when setting BaudRate.

ArmPlatformPkg: Remove UINTN cast when setting BaudRate.

SerialPortInitialize() set the BaudRate variable (type UINT64) as:
BaudRate = (UINTN)FixedPcdGet64 (PcdUartDefaultBaudRate);

This commit fixes a potential problem on ARM 32-bit builds, where the
UINTN type is defined as UINT32, by removing the cast:

BaudRate = FixedPcdGet64 (PcdUartDefaultBaudRate);

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Alexei Fedorov <alexei.fedorov@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>