firmware: arm_scpi: remove all single element structures

Both clk_get_value and sensor_value structures contains a single element
and hence needs no packing making the whole structure defination
unnecessary.

This patch gets rid of both those unnecessary structures.

Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>