docs: dt: Fix a few grammar nits in the binding/schema docs

Add missing hyphens and reword one sentence for clarity.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20211003124936.1.Idc7beddc77250bca0cfb5912b56be719d9073bc4@changeid


Signed-off-by: Rob Herring <robh@kernel.org>