The 7940 is a great phone to use with Asterisk…I have two myself!
Are you speaking of the config files for TFTP upload to the phone, or in Asterisk itself? For Asterisk, it is just like any other SIP phone and there are plenty of examples in the wikis at www.voip-info.org.
That is, of course, if you are using the SIP software on the phone. I have not tried the skinny/SCCP channels because I updated them to SIP 8.1 straight away, so I’m not sure of the config for those. There is some docs for the skinny driver included with asterisk, and downloads for other developers channel drivers for SCCP.
Cisco is giving away the SIP 8.1 software on their website. There is a link to it somewhere in the wikis. I can post the link if you desire. One caveat is that depending on the version of software on the phone, regardless of SIP or SCCP, you may need to upgrade to a lesser version first. I was able to find the SIP 6.3 software online to facilitate the upgrade. The only way to upgrade the phone is via a TFTP server.
The easiest way to config cisco phones is via TFTP upload. You put the configs onto a TFTP server (mine is on the Asterisk box itself) and either manually config the phone to get its TFTP downloads from that server, or set option 150 on a DHCP server to the IP of your chosen TFTP server.
The wiki page that baconbuttie pointed you to is the first start. Ciscos are probably more trouble than they are worth, but they really do work great once you get the hang of all the nuances of the different phones.
If you need any more help, just ask!