igc: Move the MULTI GBT AN Control Register to _regs file

MULTI GBT AN Control Register is IEEE Standard Register 7.32 (not a mask).
The right place should be in igc_reg.h file. In accordance with the
registers naming convention added IGC_' prefix.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: Avigail Dahan <avigailx.dahan@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>