mediatek: split Acer/W6 dts into W6 common dtsi and device specific dts

mediatek: split Acer/W6 dts into W6 common dtsi and device specific dts

In order to prepare OpenWrt support for other Acer W6 devices, this commit
moves all device tree components that are used by all Acer W6/W6e/W6d/W6m
routers from mt7986a-acer-predator-w6.dts to mt7986a-acer-w6-common.dtsi
(new file) and includes this dtsi file in mt7986a-acer-predator-w6.dts.

Minor changes had to be made to the device tree in order to improve clarity
and – notably – to reduce the number of dtc warnings:
- replace (obviously wrong) led@<N> gpio led entities by led-<N>
- remove unnecessary (default-state = "off") gpio led statements
- rename entity “memory” to “memory@0”
- add missing #address-cells and #address-size in /soc/mmc@11230000
- add missing #address-cells and #address-size in /soc/pcie@11280000
- introduce symbols “nvmem” and “swport0” in dtsi (referenced in dts)

The changes were checked with `diff -BEZbdtwy --suppress-common-lines ...`
(comparing two dts files created using old and new fdt-1 blobs again), see
https://github.com/openwrt/openwrt/pull/16861/#issuecomment-2455680020 .

This is the first of four commits into which the original commit was split
to make reviews easier and more targeted.

Signed-off-by: George Oldfort <openwrt@10099.de>
Link: https://github.com/openwrt/openwrt/pull/16861
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>