ARM: dts: bcm2835: Add Raspberry Pi Zero

The Raspberry Pi Zero is a minified version of model A+. It's
notable there is no PWR LED and the ACT LED is inverted.
Additionally the Pi Zero is capable of the USB peripheral mode.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Eric Anholt <eric@anholt.net>