ARM: dts: bcm283x: fix typo in mailbox address

The address of the mailbox node in the bcm283x.dtsi also has a typo.
So fix it accordingly.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Fixes: 05b682b7 ("ARM: bcm2835: dt: Add the mailbox to the device tree")
Signed-off-by: Eric Anholt <eric@anholt.net>