Cisco SIP 797xG Models [7970G, 7971G-GE]

Created a separate entry for 797x phones as they are very different from a 796x phone.

Based on the sip79x1G family, the SIP specific config options were removed (as the phone only uses the SEP xml file), and combined many of the popular features as found in http://www.voip-info.org/wiki/view/Asterisk+phone+cisco+7970+SIP.