OvmfPkg: introduce a common work area

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3429

Both the TDX and SEV support needs to reserve a page in MEMFD as a work
area. The page will contain meta data specific to the guest type.
Currently, the SEV-ES support reserves a page in MEMFD
(PcdSevEsWorkArea) for the work area. This page can be reused as a TDX
work area when Intel TDX is enabled.

Based on the discussion [1], it was agreed to rename the SevEsWorkArea
to the OvmfWorkArea, and add a header that can be used to indicate the
work area type.

[1] https://edk2.groups.io/g/devel/message/78262?p=,,,20,0,0,0::\
    created,0,SNP,20,2,0,84476064

Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>

OvmfPkg/OvmfPkg.dec

@@ -329,6 +329,18 @@
   gUefiOvmfPkgTokenSpaceGuid.PcdQemuHashTableBase|0x0|UINT32|0x47
   gUefiOvmfPkgTokenSpaceGuid.PcdQemuHashTableSize|0x0|UINT32|0x48
 
+  ## The base address and size of the work area used during the SEC
+  # phase by the SEV and TDX supports.
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfWorkAreaBase|0|UINT32|0x49
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfWorkAreaSize|0|UINT32|0x50
+
+  ## The work area contains a fixed size header in the Include/WorkArea.h.
+  # The size of this header is used early boot, and is provided through
+  # a fixed PCD. It need to be kept in sync with any changes to the
+  # header definition.
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfConfidentialComputingWorkAreaHeader|0|UINT32|0x51
+
+
 [PcdsDynamic, PcdsDynamicEx]
   gUefiOvmfPkgTokenSpaceGuid.PcdEmuVariableEvent|0|UINT64|2
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfFlashVariablesEnable|FALSE|BOOLEAN|0x10