Document the CRU IP found in Renesas RZ/G2UL SoC.

The CRU block on the RZ/G2UL SoC is identical to one found on the
RZ/G2L SoC, but it does not support parallel input.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20240605154115.263447-3-biju.das.jz@bp.renesas.comSigned-off-by: Rob Herring (Arm) <robh@kernel.org>