[ARM] 3979/1: extend the SA11x0 sched_clock implementation from 32 to 63 bit period

This provides a 63 bit clock counter guaranteed to be monotonic over a
period of 370 days instead of a clock wrap every 19.4 minutes, as long
as sched_clock() is called at least once every 9.7 minutes which
shouldn't be a problem in practice.

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