firmware: arm_scmi: Use the term 'message' instead of 'command'

In preparation to adding support for other two types of messages that
SCMI specification mentions, let's replace the term 'command' with the
correct term 'message'.

As per the specification the messages are of 3 types:
commands(synchronous or asynchronous), delayed responses and notifications.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>