OvmfPkg/Tcg2ConfigPei: introduce a signalling PPI to depex on

OvmfPkg/Tcg2ConfigPei: introduce a signalling PPI to depex on

On ARM systems, the TPM does not live at a fixed address, and so we
need the platform to discover it first. So introduce a PPI that signals
that the TPM address has been discovered and recorded in the appropriate
PCD, and make Tcg2ConfigPei depex on it when built for ARM or AARCH64.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2560
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>