OvmfPkg/PlatformPei: prepare for PcdQ35TsegMbytes becoming dynamic

OvmfPkg/PlatformPei: prepare for PcdQ35TsegMbytes becoming dynamic

In one of the next patches we'll turn PcdQ35TsegMbytes into a dynamic PCD,
to be set by PlatformPei. Introduce the Q35TsegMbytesInitialization()
function and the "mQ35TsegMbytes" global variable to support this.

Q35TsegMbytesInitialization() manages the PCD and caches its final value
into "mQ35TsegMbytes". Call Q35TsegMbytesInitialization() from
InitializePlatform() just in time for the current PCD consumers,
PublishPeiMemory(), InitializeRamRegions() and QemuInitializeRam() --
which is called from InitializeRamRegions() -- to be rebased on top of
"mQ35TsegMbytes".

Call Q35TsegMbytesInitialization() only when PcdSmmSmramRequire is TRUE,
given that PcdQ35TsegMbytes is consumed in that case only.

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: Jordan Justen <jordan.l.justen@intel.com>