SBI: Attestation Extension

This patch add an attestation and measurement SBI extension (`ATST`).

The `ATST` extension defines a set of functions for SBI implementations
to provide attestation primitives, mostly related to the confidential
computing remote attestation flows.

Besides allowing SBI callers to request for attestation certificates,
this extension also give them the ability to dynamically extend the TCB
measurements set and read measurement registers back.

SBI implementations must expose their attestation and measurement
capabilities through the `ATST` FID #0: `get_capabilities`.

Signed-off-by: Samuel Ortiz <[email protected]>