device: make device dependencies optional

Device dependencies are not always required, so make them optional via
CONFIG_DEVICE_DEPS. When enabled, the gen_device_deps script will run so
that dependencies are collected and part of the final image. Related
APIs will be also made available. Since device dependencies are used in
just a few places (power domains), disable the feature by default. When
not enabled, a second linking pass will not be required.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>