e1000: introduce DPGFR register

Defined DPGFR, Dynamic Power Gate Force Control Register.

Signed-off-by: Vitaly Lifshits <vitaly.lifshits@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 (1469e5aceffbdcebe834292aadb40b1bd1602867)
MFC after: 1 week

(cherry picked from commit 7fb2111413c799414c86d7bfdcc72bc1c6302726)