Add nps400 to the list of acceptable values for the -mcpu command line
switch, and to the .cpu directive.
I've added an extra cross reference from -mcpu to .cpu to improve
navigation of the documentation.
gas/ChangeLog:
* doc/c-arc.texi (ARC Options): Add nps400 to list of valus for
-mcpu. Add cross reference to .cpu directive from -mcpu option.
(ARC Directives): Add NPS400 to .cpu directive list.