ice: use bitfields instead of unions for CGU regs

ice: use bitfields instead of unions for CGU regs

Switch from unions with bitfield structs to definitions with bitfield
masks. This is necessary, because some registers have different
field definitions or even use a different register for the same fields
based on HW type.

Remove unused register fields.

Reviewed-by: Milena Olech <milena.olech@intel.com>
Signed-off-by: Karol Kolacinski <karol.kolacinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>