power: Add substate-id property for a power state

There are platforms that have multiple states that maps to a specific
Zephyr power state. To accommodate this sort of situation this commit
adds an additional property to a power state that can be used by the
platform.

The power state now consists of two properties, a category and a
substate-id. The former property is the current power state.

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