clocksource: pxa: Add device-tree support for PXA timer

Add device-tree support to PXA platforms.
The driver still needs to maintain backward non device-tree
compatibility as well, which implies :
 - a non device-tree init function
 - a static registers base address in the driver

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>