pch_gbe: support ML7831 IOH

Support new device OKI SEMICONDUCTOR ML7831 IOH(Input/Output Hub)

ML7831 is for general purpose use.
ML7831 is companion chip for Intel Atom E6xx series.
ML7831 is completely compatible for Intel EG20T PCH.

Signed-off-by: Toshiharu Okada <toshiharu-linux@dsn.okisemi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>