OvmfPkg: make PcdQ35TsegMbytes dynamic

OvmfPkg: make PcdQ35TsegMbytes dynamic

We can now make PcdQ35TsegMbytes dynamic, in preparation for the extended
TSEG size feature. At the moment we only move the declaration in
OvmfPkg.dec from [PcdsFixedAtBuild] to [PcdsDynamic, PcdsDynamicEx], and
provide the dynamic defaults (with the same value, 8) in the DSC files if
SMM_REQUIRE is TRUE.

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>