sample: demand_paging: add a demo about ondemand section usage

This sample demonstrates how demand paging can be leveraged to deal with
firmware bigger than the available RAM if XIP is not possible. Select
code can be tagged to be loaded into memory on demand, and also be
automatically evicted for more code to be executed when memory is
exhausted.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>