document clocksources

Document the available clocksources per platform and move clocksource= into
the correct (alpha) location in the file.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>