firmware: arm_scpi: improve struct sensor_value

lo_val and hi_val together in this order are a little endian 64 bit value.
Therefore we can simplify struct sensor_value and the code by defining
it as a __le64 value and by using le64_to_cpu.

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