samples: modbus: update/fix samples.yaml configs

Update the sample.yaml files for the modbus samples:
- depends_on entries should just be space separated, drop the comma
- add the platform referenced in the documentation to platform_allow
- replace the deprecated dt_compat_enabled_with_alias with
  dt_enabled_alias_with_parent_compat

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>