hardware_gen: Refactor calculating device regions

- Device.regions() now just calculates memory regions without splitting
them into user and kernel groups.
- Config.split_regions() now calls Device.regions() and performs the
splitting if the device is a kernel device, otherwise returns the
original regions.
- Config.split_regions() is now only used in a context when dealing with
kernel devices, otherwise Device.regions() can be called to return only
informatioin extracted from the device tree.