Backport BCM270X_DT: Rename Pi Zero W DT files

This patch needed to be backported to series 4.14.y-rt because it is also supported in meta-raspberrypi for Yocto Project.
Unfortunately, the rename affected the correct building of the 4.14 series kernel, which now results broken.
Applying the same renames on the 4.14.y-rt series would fix the issue for those still using that kernel version.

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>
Signed-off-by: Francesco Giancane <francescogiancane8@gmail.com>