firmware: arm_scpi: use FIELD_GET/_PREP to simplify macro definitions

Macro definitions can be simplified by making use of the FIELD_GET/_PREP
bitfield macros.

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