iwlwifi: mvm: use correct GP2 register address for 22000 family

The device time register address has changed for 22000 devices.
Add a util function for getting the GP2 time and use the correct
register address depending on the device family.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>