OvmfPkg/SecMain: register GHCB gpa for the SEV-SNP guest

OvmfPkg/SecMain: register GHCB gpa for the SEV-SNP guest

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

The SEV-SNP guest requires that GHCB GPA must be registered before using.
See the GHCB specification section 2.3.2 for more details.

Cc: Michael Roth <michael.roth@amd.com>
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>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>