intel: adsp: Simplify PM

Both idle and suspend states were just being used to set the cpu
idle. That is not necessary, if the pm policy does not find a suitable
power state the kernel automatically calls k_cpu_idle().

This remove unnecessary code and the weirdness of having
min-residency-us set to 0 and other arbitrary values.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>