dt/bindings: serial: bcm2835: add binding documentation for bcm2835-aux-uart

Add binding documentation for the bcm2835-aux-uart driver.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>