samples: rust: platform: Add property child and reference args examples

Add some example usage of the device property methods for reading
DT/ACPI/swnode child nodes and reference args.

Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>
Link: https://lore.kernel.org/r/20250616154511.1862909-4-remo@buenzli.dev


[ Convert 'child@{0,1}' to 'child-{0,1}'; skip child nodes without
  'compatible' property in of_unittest_platform_populate() as proposed
  by Rob Herring. - Danilo]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>