tests: fs: fat_fs_api: Add NXP platforms for filesystem.fat.ram.api

- Adds NXP platforms for the filesystem.fat.ram.api test case.
- Renames prj_native_ram.conf to prj_ram.conf,
  as it can be used by all platforms.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>