device: simplify structure of handles array

Force the inclusion of a `DEVICE_HANDLE_SEP` at the end of the
devicetree dependency section of the array. This lets us simplify the
implementation of `device_required_handles_get`, as there is only one
symbol the section ends with.

This does not use any extra ROM as the array is padded out to the
original size with `DEVICE_HANDLE_ENDS` anyway.

Also adds a description of the array format where the array is
instantiated.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>
2 files changed