Add RSPI binding documentation for Renesas RZ/G2UL SoC.

RSPI block is identical to one found on RZ/A, so no driver changes are
required. The fallback compatible string "renesas,rspi-rz" will be used
on RZ/G2UL.
Signed-off-by: NBiju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Acked-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220501082150.24662-1-biju.das.jz@bp.renesas.comSigned-off-by: NMark Brown <broonie@kernel.org>