timer: xtensa: restore smp_timer_init()

The smp_timer_init() was removed during timer re-write.
This results in undefined references error during compilation
when CONFIG_SMP=y. So add it back so we can compile for SMP.
The logic is updated from the previous version to the latest
in the driver.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>