gas/doc/arc: Add nps400 references into the documentation

gas/doc/arc: Add nps400 references into the documentation

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.