OvmfPkg: add PEIM for providing TSEG-as-SMRAM during PEI

OvmfPkg: add PEIM for providing TSEG-as-SMRAM during PEI

"MdeModulePkg/Library/SmmLockBoxLib/SmmLockBoxPeiLib.inf" is the
LockBoxLib instance with SMRAM access for the PEI phase.

Said library instance must, and can, access the LockBox data in SMRAM
directly if it is invoked before SMBASE relocation / SMI handler
installation. In that case, it only needs PEI_SMM_ACCESS_PPI from the
platform, and it doesn't depend on EFI_PEI_SMM_COMMUNICATION_PPI.

OVMF satisfies the description in SVN r18823 ("MdeModulePkg:
SmmLockBoxPeiLib: work without EFI_PEI_SMM_COMMUNICATION_PPI"): in OVMF,
only S3Resume2Pei links against SmmLockBoxPeiLib.

Therefore, introduce a PEIM that produces the PEI_SMM_ACCESS_PPI
interface, enabling SmmLockBoxPeiLib to work; we can omit including
"UefiCpuPkg/PiSmmCommunication/PiSmmCommunicationPei.inf".

The load / installation order of S3Resume2Pei and SmmAccessPei is
indifferent. SmmAccessPei produces the gEfiAcpiVariableGuid HOB during its
installation (which happens during PEI), but S3Resume2Pei accesses the HOB
only when the DXE IPL calls its S3RestoreConfig2 PPI member, as last act
of PEI.

MCH_SMRAM_D_LCK and MCH_ESMRAMC_T_EN are masked out the way they are, in
SmmAccessPeiEntryPoint() and SmramAccessOpen() respectively, in order to
prevent VS20xx from warning about the (otherwise fully intentional)
truncation in the UINT8 casts. (Warnings reported by Michael Kinney.)

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19040 6f19259b-4bc3-4df7-8a09-765794883524