staging: mt7621-dts: Replace spaces with tabs in indentation

Replace spaces with tabs in indentation.
Issue found with checkpatch.

Signed-off-by: Nishka Dasgupta <nishka.dasgupta_ug18@ashoka.edu.in>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>