ARM: dts: stm32: Deduplicate DSI node on stm32mp15

All boards using the DSI node duplicate the same pattern common pattern
in board DTs, that pattern is ports with endpoint labels and the same
in-SoC regulator connection. Move that common pattern into stm32mp157.dtsi
instead.

The two boards which do define panel@0 directly in the DSI bridge node now
have #address-cells/#size-cells in their board DT instead of it being in
stm32mp157.dtsi and activated incorrectly for all boards, even the ones
which use e.g. another DSI-to-something bridge.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Raphaël Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>