refactor(fvp): use RSS API to retrieve attestation token and key

Retrieved the platform attestation token and delegated realm attestation
key through the PSA delegated attestation layer.

Even though FVP doesn't support RSS hardware today, it can still
leverage the RSS implementation of these PSA interfaces in their mocking
form (see PLAT_RSS_NOT_SUPPORTED).

Therefore, platform APIs now call these PSA interfaces instead of
directly providing these hardcoded values.

Change-Id: I31d0ca58f6f1a444f513d954da4e3e67757321ad
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>