device: s/gen_handles/gen_device_deps

Rename the gen_handles script and all of its references/associated files
to gen_device_deps. The new new makes things more clear, because the
script just take care of generating, for each device, an array of device
dependencies. While device handles are used internally to store this
information, it is in reality an implementation detail.

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