[rescue] NVRAM replacement on sun4c machines

[Andy Wallis]

This should add a few more datapoints for the NVRAM replacement FAQ and
generail sun4c recovery.

I have put a pair of Maxim (formally Dallas Semiconductor) DS1642-70s into 
a SLC and IPC. Both NVRAM replacments work without any problems in their
respective machines.

This should help remove the problem that the changes made to the SGS-Thompson
(formerly Mostek) M48T02 chips had on the sun4c machines.

-Andy Wallis