Freshly introduced NEGOTIATE_PROTOCOL_VERSION allows the agent to ascertain
upfront if a specific protocol(usually older) version is supported by the
platform.

It is used by the agent in case the platform has advertised the support of
a newer protocol version than the latest version supported by the agent,
since backward compatibility cannot be automatically assumed.

Emit a warning about possible incompatibility when negotiation was not
possible or just print the successfully negotiated protocol.
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240214183006.3403207-3-cristian.marussi@arm.comSigned-off-by: Sudeep Holla <sudeep.holla@arm.com>