d/rules: Call make clean between each subplatforms rebuild

https://phabricator.apertis.org/T8220

For each subplatform, make is called using the same PLAT variable. If the build is not cleaned between each call, objects will remain the same, without rebuilding them.

This issue is producing all subplatforms targets to be the same for each platform, e.g.: imx8mn and imx8mn_uart4;