ArmPlatformPkg/PL031RealTimeClockLib: depend on gEfiCpuArchProtocolGuid

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.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: Supreeth Venkatesh <Supreeth.Venkatesh@arm.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Steve Capper <steve.capper@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>