drivers/timer/xtensa_sys_timer: Correctly declare ISR

The existing hand-written interrupt code is manually calling the timer
ISR, which is sort of silly and about to be replaced.  Correctly
declare the ISR with IRQ_CONNECT() so that a conventional interrupt
handling implementation can find it.  With current code this is a
noop.

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