[ARM] 3981/1: sched_clock for PXA2xx

Here's a 63-bit implementation of shed_clock() for PXA2xx.  The actual
period depends on the value of CLOCK_TICK_RATE and whether or not
reduced scaling factors were provided for it.

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