soc: xtensa: intel_adsp: cavs: fix PM hooks guards

The PM hooks were guarded with CONFIG_PM_POLICY_CUSTOM, however, they
need to be guarded (if file is always compiled) with CONFIG_PM. In fact,
CONFIG_PM_POLICY_CUSTOM requires to implement a custom policy hook,
something this module did not provide.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>