dt-binding: rng: Add interrupt property for BCM2835

As Sylvain Garrigues pointed out the BCM2835 RNG block has an
interrupt. Also in preparation for Florian's upcoming patch series
add this property to the binding. While we are at it fix the
indenting of the example.

Link: https://github.com/raspberrypi/linux/issues/2195


CC: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>