drivers/xtensa_sys_timer: Change default timer ID

Use index zero, not one.  The Xtensa tools emit the timers in priority
order, and as mentioned in the kconfig warnings using high priority
timers doesn't work.  This also makes room for using software
interrupts that can preempt a timer interrupt for test purposes.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>