feat(lib/psa): mock PSA APIs

Introduce PLAT_RSS_NOT_SUPPORTED build config to
provide a mocked version of PSA APIs. The goal is
to test the RSS backend based measured boot and
attestation token request integration on such
a platform (AEM FVP) where RSS is otherwise
unsupported. The mocked PSA API version does
not send a request to the RSS, it only returns
with success and hard-coded values.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: Ice8d174adf828c1df08fc589f0e17abd1e382a4d