docs: dt: submitting-patches: Fix grammar in subsystem section

Reword the subsystem bindings section to make sense, from a grammatical
point of view.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20210309130650.2318419-1-geert+renesas@glider.be


Signed-off-by: Jonathan Corbet <corbet@lwn.net>