i82801gx: Add setting for C4onC3 mode

i82801gx: Add setting for C4onC3 mode

If this bit is set, ich7 will enter C4 mode if possible instead of
C3. See ich7 specification (LPC controller, Power management control
registers) for more details.

Change-Id: I352cccdbc51ff6269f153a4542c7ee1df0c01d22
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/329
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>