xtensa: return to idle() when booting sporadically

To power down secondary cores on cAVS 2.5 platforms the primary core
enables the power-saving mode for the respective secondary core and
waits until that core enters idle() and executes the waiti
instruction at which point the core should enter a lower-power mode.
However, that core can then also automatically wake up and execute
its reset path if, e.g. an interrupt is delivered to it. However, it
isn't entirely clear which events are able to wake up cores from that
state. Disabling interrupts on the interrupt controller didn't seem
to prevent that from happening completely. In particular a specific
ADL notebook seems to be susceptible to this problem. Checking for
such sporadic boots and returning to idle fixes the problem.

BugLink: https://github.com/zephyrproject-rtos/zephyr/issues/46372
BugLink: https://github.com/thesofproject/sof/issues/5733


Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>