firmware: arm_scmi: Reorder some functions to avoid forward declarations

Re-shuffling few functions to keep definitions and their usages close.
This is also needed to avoid too many unnecessary forward declarations
while adding new features(delayed response and notifications).

Keeping this separate to avoid mixing up of these trivial change that
doesn't affect functionality into the ones that does.

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