spi: dt-bindings: Document the RZ/V2H(P) RSPI

Add dt-bindings for the RSPI IP found inside the Renesas RZ/V2H(P)
SoC.

Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>

Link: https://patch.msgid.link/20250704162036.468765-2-fabrizio.castro.jz@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>