BCM270X_DT: Rename Pi Zero W DT files

The downtream Pi Zero W dts file uses the digit 0, whereas upstream
chose to spell it out - "zero-w". The firmware has, for a long time,
looked for bcm2708-rpi-zero-w.dtb first before falling back to the
numerical version. Therefore it is better to follow upstream and
make the switch to "bcm2708-rpi-zero-w".

At the same time, remove some overrides that duplicate values
inherited from the shared .dtsi files.

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