ArmVirtPkg: Use dynamic PCD to set the SMCCC conduit

On ARM systems, whether SMC or HVC instructions need to be used to issue
monitor calls is typically dependent on the exception level, but there
are also cases where EL1 might use SMC instructions, so there is no hard
and fast rule.

For ArmVirtQemu, this does depend strictly on the exception level, so
set the default to HVC (for EL1 execution) and override it to SMC when
booted at EL2.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>

Committed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Doug Flick [MSFT] <doug.edk2@gmail.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

ArmVirtPkg/ArmVirtQemuKernel.dsc

@@ -203,6 +203,8 @@
   gArmTokenSpaceGuid.PcdFdBaseAddress|0x0
   gArmTokenSpaceGuid.PcdFvBaseAddress|0x0
 
+  gArmTokenSpaceGuid.PcdMonitorConduitHvc|TRUE
+
 [PcdsDynamicDefault.common]
   gEfiMdePkgTokenSpaceGuid.PcdPlatformBootTimeOut|3