ramips: dts: use generic node name for flash

In DTS Checklist[1] we're now demanding proper generic node names, as
the name of a node should reflect the function of the device and use
generic name for that[2]. Everybody seems to be copy&pasting from DTS
files available in the repository today, so let's unify that naming
there as well and provide proper examples.

While at it, remove unused m25p80 label.

Tested on rt5350 (for spi-nor) and rt3662 (for cfi-flash).

1. https://openwrt.org/submitting-patches#dts_checklist
2. https://github.com/devicetree-org/devicetree-specification/blob/master/source/devicetree-basics.rst#generic-names-recommendation

Signed-off-by: Sungbo Eo <mans0n@gorani.run>