[ARM] 3980/1: extend the ARM Versatile sched_clock implementation from 32 to 63 bit

 period

This provides a 63 bit clock counter guaranteed to be monotonic over a
period of 35583 days instead of a clock wrap every 179 seconds, as long
as sched_clock() is called at least once every 89 seconds.  This should
not be a problem in practice, although a kernel timer could be scheduled
every 80 seconds for example simply to call sched_clock() making sure
top bits are always synchronized if need be.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>