StandaloneMmPkg/Library: Install Variable Arch Protocol

StandaloneMmPkg/Library: Install Variable Arch Protocol

In a system implementing the variable store in MM, there are no variable
arch protocol and variable write arch protocol installed into the
DXE_SMM protocol database. On such systems, it is not required to
locate these protocols by the DXE runtime variable drivers because
it can be assumed that these protocols are already installed in the
MM context. But then such an implementation will deviate from the
existing traditional MM based variable driver implementation.

So in order to maintain consistency with the traditional MM variable
driver implementation, allow platforms to install these protocols into
the DXE protocol database but these protocol will not be consumed
by non-secure variable service runtime driver.

The Platform which uses StandaloneMM based secure variable storage
have to include this library

Example
In edk2-platforms/Platform/ARM/SgiPkg/SgiPlatform.dsc

  ArmPkg/Drivers/MmCommunicationDxe/MmCommunication.inf {
    <LibraryClasses>
      NULL|StandaloneMmPkg/Library/VariableMmDependency/VariableMmDependency.inf
  }

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>
Acked-by "Yao, Jiewen" <jiewen.yao@intel.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>