cavs: disable timer interrupt on cpu power off

Since the timer interrupt is delivered to all cores at the same
time, it needs to be disabled (or masked) to prevent it from
waking a suspended CPU core.

Note that there is no need to re-enable (unmask) timer
interrupt separately as it is being taken care of in
smp_timer_init() when the CPU re-inits itself.

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