documentation: update sysrq.txt magic sysrq keys

Update Documentation/sysrq.txt magic sysrq keys:

 - 'g' is for kgdb (not arch-specific);
 - add 2 new uses for 'v', remove the Voyager info;
 - add 'y' info (SPARC-64 specific);

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "James E.J. Bottomley" <James.Bottomley@suse.de>
Cc: David Airlie <airlied@linux.ie>
Acked-by: Alexander Shishkin <virtuoso@slind.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>