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>

Documentation/sysrq.txt

@@ -75,7 +75,7 @@ On all -  write a character to /proc/sysrq-trigger.  e.g.:
 
 'f'	- Will call oom_kill to kill a memory hog process.
 
-'g'	- Used by kgdb on ppc and sh platforms.
+'g'	- Used by kgdb (kernel debugger)
 
 'h'     - Will display help (actually any other key than those listed
           here will display help. but 'h' is easy to remember :-)
@@ -110,12 +110,15 @@ On all -  write a character to /proc/sysrq-trigger.  e.g.:
 
 'u'     - Will attempt to remount all mounted filesystems read-only.
 
-'v'	- Dumps Voyager SMP processor info to your console.
+'v'	- Forcefully restores framebuffer console
+'v'	- Causes ETM buffer dump [ARM-specific]
 
 'w'	- Dumps tasks that are in uninterruptable (blocked) state.
 
 'x'	- Used by xmon interface on ppc/powerpc platforms.
 
+'y'	- Show global CPU Registers [SPARC-64 specific]
+
 'z'	- Dump the ftrace buffer
 
 '0'-'9' - Sets the console log level, controlling which kernel messages