arm64: dts: renesas: Prepare AA1024XD12 panel .dtsi for overlay support

The Mitsubishi AA1024XD12 panel can be used for R-Car Gen2 and Gen3
boards as an optional external panel. It is described in the
arm/boot/dts/r8a77xx-aa104xd12-panel.dtsi file as a direct child of the
DT root node. This allows including r8a77xx-aa104xd12-panel.dtsi in
board device trees, with other minor modifications, to enable the panel.

This is however not how external components should be modelled. Instead
of modifying the board device tree to enable the panel, it should be
compiled as a DT overlay, to be loaded by the boot loader.

Prepare the r8a77xx-aa104xd12-panel.dtsi file for this usage by
declaring a panel node only, without hardcoding its path. Overlay
sources can then include r8a77xx-aa104xd12-panel.dtsi where appropriate.

This change doesn't cause any regression as r8a77xx-aa104xd12-panel.dtsi
is currently unused. As overlay support for this panel has only been
tested with Gen3 hardware, and Gen2 support will require more
development, move the file to arch/arm64/boot/dts/renesas/.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20211229193135.28767-2-laurent.pinchart+renesas@ideasonboard.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>