ArmPkg/ArmPkg.dsc: Add missing components

ArmPkg/ArmPkg.dsc: Add missing components

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2574

The following components are currently missing from the [Components]
section of ArmPkg.dsc:
  * ArmPkg/Drivers/ArmCrashDumpDxe/ArmCrashDumpDxe.inf
  * ArmPkg/Drivers/MmCommunicationDxe/MmCommunication.inf
  * ArmPkg/Library/ArmMtlNullLib/ArmMtlNullLib.inf
  * ArmPkg/Library/ArmSmcPsciResetSystemLib/ArmSmcPsciResetSystemLib.inf

This commit includes the components in the package DSC build.

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>