firmware: arm_scpi: remove dvfs_get packed structure

dvfs_get packed structure is used to read the DVFS/OPP index from the
firmware. It just contains a single byte that needs no packing making
the whole structure defination unnecessary.

This patch replaces the unnecessary dvfs_get packed structure with an
unsigned byte.

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