SBI spec: return code for non-available extension

[Heinrich Schuchardt]

Software may by fault invoke an extension which is not available.

OpenSBI actually returns SBI_ERR_NOT_SUPPORTED from sbi_ecall_handler()
in this case.

This is a reasonable choice but I could not find any basis in the SBI
specification. The handling of an ecall for an unsupported EID is not
described.

I think this gap should be closed in the spec.

Best regards

Heinrich