pm: policy: Fix possible undefined reference

When PM_POLICY_DEVICE_CONSTRAINTS is enabled, we create a list of
devices with the property “zephyr,disabling-power-states” and their
respective states to lock. However, if a device is declared but not
built (e.g., due to a Kconfig option), we have an undefined reference.

This PR changes the code to use the device name instead of the
reference. This increases lookup time (which could be cached at
runtime), and we still have an entry for a non-existent device.

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