ArmPlatformPkg/PL031RealTimeClockLib: depend on gEfiCpuArchProtocolGuid
The RealTimeClockLib class is declared under EmbeddedPkg, so that
platforms can provide the internals for the
EmbeddedPkg/RealTimeClockRuntimeDxe driver. In turn the driver produces
the Real Time Clock Arch Protocol, without which UEFI drivers cannot be
dispatched.
The PL031RealTimeClockLib instance calls gDS->SetMemorySpaceAttributes()
in the LibRtcInitialize() public function. This DXE service depends on the
CPU Arch Protocol. Add it to the depex.