firmware: arm_scpi: improve handling of protocol and firmware version subfields

By using FIELD_GET and proper masks we can avoid quite some shifting
and masking macro magic and make the code better readable.

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