microblaze: Added fdt chosen capability for timer

This lets a dts author flag a particular timer in the system as the system timer.
If the chosen node contains a "system-timer=<&foo>" entry than that handle will
be used to determine the system timer. In no such entry exists then the first
found timer will be used (current behaviour).

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>