OvmfPkg/PlatformPei: honor extended TSEG in PcdQ35TsegMbytes if available

OvmfPkg/PlatformPei: honor extended TSEG in PcdQ35TsegMbytes if available

Recognize an extended TSEG when available in
Q35TsegMbytesInitialization(), and set both PcdQ35TsegMbytes (for
OvmfPkg/SmmAccess) and "mQ35TsegMbytes" (for PlatformPei's own use)
accordingly. The new logic interfaces with the QEMU feature added in QEMU
commit 2f295167e0c4 ("q35/mch: implement extended TSEG sizes",
2017-06-08).

At this point we have to explicitly restrict Q35TsegMbytesInitialization()
to the Q35 board, but that's OK, because Q35TsegMbytesInitialization() is
only called when PcdSmmSmramRequire is set, and for that Q35 is already an
enforced requirement.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>