OvmfPkg: Define a new PPI GUID to signal TPM MMIO accessability

OvmfPkg: Define a new PPI GUID to signal TPM MMIO accessability

Define a new PPI GUID that is to be used as a signal of when it is safe
to access the TPM MMIO range. This is needed so that, when SEV is active,
the MMIO range can be mapped unencrypted before it is accessed.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <03e292339273721724c8b14605cfe9d7bbe45a71.1619716333.git.thomas.lendacky@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>