ramips: mt7621: Move common DNA EX400 defs to dtsi

ramips: mt7621: Move common DNA EX400 defs to dtsi

Move common definitions for DNA Valokuitu Plus EX400 to a dtsi include.
This is in preparation of adding the non-branded variant of the device
produced by Genexis / Inteno in the next commit. The device with DNA
branding differs in the LED labling on the device.

Signed-off-by: Andreas Gnau <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/17551
(cherry picked from commit e8603f3b5a15856fccee2e02a5406ca9c8b2ba1d)
Link: https://github.com/openwrt/openwrt/pull/18238
Signed-off-by: Hauke Mehrtens <[email protected]>