pm: device_runtime: PM unsupported is not an error

Returning an error code when PM is not supported for a device only makes
writing drivers harder, as instead of checking for failures with `< 0`,
they also need to check for `-ENOTSUP`.

From the point of view of a driver, an `-ENOTSUP` return value is
equivalent to success, as if the device it is trying to turn on doesn't
support PM, by definition it is already enabled. This is equivalent to
the API behaviour when `CONFIG_PM_DEVICE_RUNTIME=n`.

Whether a device supports PM or not can still be determined at runtime
by inspecting the return code of `pm_device_runtime_enable` if
necessary.

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