ARM: dts: bcm283x: Correct mailbox register sizes

The size field in a Device Tree "reg" property is encoded in bytes, not
words.

Fixes: 614fa221 ("ARM: dts: bcm2835: Add VCHIQ node to the Raspberry Pi boards. (v3)")
Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>