ramips: split Teltonika RUT9M SPI-NOR layout into separate dtsi

ramips: split Teltonika RUT9M SPI-NOR layout into separate dtsi

Introduce mt7628an_teltonika_rut9m.dtsi to hold the SPI-NOR flash
definition and partition layout specific to the RUT9M series.

This refactor prepares for supporting the Tetonika RUT976
(part of the RUTE series), which shares most hardware with
RUT9M devices but has a different flash layout.

Update RUT9x1 and RUT9x6 DTS files to include the new dtsi.
Existing RUT9M devices remain unchanged.

Signed-off-by: Til Kaiser <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20933
Signed-off-by: Hauke Mehrtens <[email protected]>