Add RSPI binding documentation for Renesas RZ/G2L 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/G2L.
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20211118031041.2312-2-prabhakar.mahadev-lad.rj@bp.renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>