[PATCH] x86_64: Add useful constants to time.h

In timekeeping code, one often does need to use conversion constants. Naming
these leads to code that's easier to understand, showing the reader between
which units the conversion is made.

Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>