Patch from Tony Lindgren

This patch was suggested by RMK, and adds a warning on the accuracy
of timekeeping when using dynamic tick on some platforms. Depending
on the timer implementation, dynamic tick may affect the accuracy of
timekeeping.
Currently at least OMAP is known to have accurate timekeeping with
dynamic tick.
Signed-off-by: NTony Lindgren <tony@atomide.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>