BCM270X_DT: Create bcm2708-rpi-zero.dts

The Pi Zero deserves a dedicated .dtb file - sharing the b-plus .dtb
has been observed to cause an issue with the MAC address of some
Ethernet dongles.

See: https://github.com/raspberrypi/linux/issues/2990



Signed-off-by: Phil Elwell <phil@raspberrypi.org>