UefiCpuPkg/SecCore: get AllSecPpiList after SecPlatformMain.

UefiCpuPkg/SecCore: get AllSecPpiList after SecPlatformMain.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2136

SecPlatformMain is a platform hook function which let platform does
some update. Some platform may adjust SecCoreData->PeiTemporaryRamBase
which caused former saved AllSecPpiList variable invalid.

This patch update the logic to get AllSecPpiList after SecPlatformMain.
If SecPlatformMain() returns no platform-specific PPI list, then there
is nothing to merge, so we don't need "AllSecPpiList" at all.

Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>