ArmPkg: PCD to select conduit for monitor calls

Define a PCD 'PcdMonitorConduitHvc' to select the conduit to use for
monitor calls. PcdMonitorConduitHvc is defined as FALSE by default,
meaning the SMC conduit is enabled as default.

Adding PcdMonitorConduitHvc allows selection of HVC conduit to be used
by virtual firmware implementations.

Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>