PcAtChipsetPkg/SerialIoLib: Allow configurable UART IO Port Base Address

Today's implementation used a hardcoded value for the I/O Port
Base Address. This creates a problem for platform configurability.
If a platform needs to use a different I/O port address, then
it requires modification of a source file.

Create gPcAtChipsetPkgTokenSpaceGuid.PcdUartIoPortBaseAddress
as a FixedAtBuild pcd scoped to PcAtChipsetPkg, with default
value for 0x3f8 to match existing value, and update
code to use PCD.

Signed-off-by: Aaron Pop <aaronpop@microsoft.com>