device: provide internal access to static device array

Device objects in Zephyr are currently placed into an array by linker
scripts, making it easy to iterate over all devices if the array
address and size can be obtained.  This has applications in device
power management, but the existing API for this was available only
when that feature was enabled.  It also uses a signed type to hold the
device count.

Provide a new API that is generally available, but marked as internal
since normally applications should not iterate over all devices.  Mark
the PM API approach deprecated.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>