[ARM] 3135/1: harden SA11x0 and PXA2xx timer init code

Patch from Nicolas Pitre

Make it completely deterministic and leave nothing to chance
(even if it had at worst 0.001% probability of failing).

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