arm: dts: mt7623: move display nodes to separate mt7623n.dtsi

mt7623a has no graphics support so move nodes from generic mt7623.dtsi
to mt7623n.dtsi

Fixes: 1f6ed224

 ("arm: dts: mt7623: add Mali-450 device node")
Suggested-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Link: https://lore.kernel.org/r/20200904110002.88966-3-linux@fw-web.de
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>