e1000: expose FEXTNVM registers and masks

Adding defines for FEXTNVM8 and FEXTNVM12 registers with new masks for
future use.

Signed-off-by: Nir Efrati <nir.efrati@intel.com>
Signed-off-by: Guinan Sun <guinanx.sun@intel.com>
Reviewed-by: Wei Zhao <wei.zhao1@intel.com>

Approved by: imp
Obtained from: DPDK (6d208ec099cd870a73c6b444b350a82c7a26c5e4)
MFC after: 1 week