DT: binding: bcm2835-mbox: fix address typo in example

The address of the mailbox node in the example has a typo.
So fix it accordingly.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: d4b5c782 ("dt/bindings: Add binding for the BCM2835 mailbox driver")
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Eric Anholt <eric@anholt.net>