firmware: arm_scpi: improve struct dvfs_info to make code better readable

Making the header subfields members of struct dvfs_info allows to make
the code better readable and avoids some macro magic.

In addition remove a useless statement using info->latency.

Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>