LX2160aCex7: Link VariableMmDependency into MmCommunicationOpteeDxe

VariableSmmRuntimeDxe registers a callback on gEfiSmmVariableProtocolGuid
and only installs the Variable Arch Protocol when that signal fires. On
x86, PiSmmIpl/StandaloneMmIpl bridges this protocol from SMM/MM to DXE.
On ARM with OP-TEE StandaloneMm, no such bridge exists.

VariableMmDependency is a DXE NULL library whose constructor installs
gEfiSmmVariableProtocolGuid and gSmmVariableWriteGuid as stub signal
protocols on the DXE side. Upstream ARM platforms (VExpress-FVP, Juno)
and OVMF link it into their MM Communication driver for exactly this
reason.

Without this, VariableSmmRuntimeDxe loads but never installs Variable
Arch Protocol, causing DxeCore to ASSERT at BdsEntry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lizthegrey

Platform/SolidRun/LX2160aCex7/LX2160aCex7.dsc

@@ -243,7 +243,10 @@
   # Architectural Protocols
   #
 !if $(SECURE_BOOT_ENABLE) == TRUE
-  ArmPkg/Drivers/MmCommunicationOpteeDxe/MmCommunication.inf
+  ArmPkg/Drivers/MmCommunicationOpteeDxe/MmCommunication.inf {
+    <LibraryClasses>
+      NULL|StandaloneMmPkg/Library/VariableMmDependency/VariableMmDependency.inf
+  }
   MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf {
     <LibraryClasses>
       MmUnblockMemoryLib|MdePkg/Library/MmUnblockMemoryLib/MmUnblockMemoryLibNull.inf