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>

PcAtChipsetPkg/Library/SerialIoLib/SerialIoLib.inf

@@ -17,6 +17,7 @@
 
 [Packages]
   MdePkg/MdePkg.dec
+  PcAtChipsetPkg/PcAtChipsetPkg.dec
 
 [LibraryClasses]
   BaseLib
@@ -25,3 +26,5 @@
 [Sources]
   SerialPortLib.c
 
+[Pcd]
+  gPcAtChipsetPkgTokenSpaceGuid.PcdUartIoPortBaseAddress

PcAtChipsetPkg/Library/SerialIoLib/SerialPortLib.c

@@ -43,7 +43,7 @@
 // ---------------------------------------------
 // UART Settings
 // ---------------------------------------------
-UINT16  gUartBase = 0x3F8;
+UINT16  gUartBase = FixedPcdGet16 (PcdUartIoPortBaseAddress);
 UINTN   gBps      = 115200;
 UINT8   gData     = 8;
 UINT8   gStop     = 1;

PcAtChipsetPkg/PcAtChipsetPkg.dec

@@ -164,5 +164,10 @@
   # @Prompt RTC Update Timeout Value.
   gPcAtChipsetPkgTokenSpaceGuid.PcdRealTimeClockUpdateTimeout|100000|UINT32|0x00000020
 
+[PcdsFixedAtBuild]
+  ## Defines the UART base address.
+  # @Prompt UART IO Port Base Address
+  gPcAtChipsetPkgTokenSpaceGuid.PcdUartIoPortBaseAddress|0x3F8|UINT16|0x00000024
+
 [UserExtensions.TianoCore."ExtraFiles"]
   PcAtChipsetPkgExtra.uni