dt/bindings: add documentation on standard property unit suffixes

A common review comment is to add units to DT property names. The naming
convention is inconsistent unfortunately although there are clear
majorities in use already. This makes it hard to remember the exact name
preferred, so I'm documenting this to provide an easy point of
reference.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>