dt-bindings: clock: r9a07g043-cpg: Annotate RZ/G2UL-only core clocks

The M2 (CRU main clock), M3 (LCDC Video Clock), and AT (Cortex-A55 Debug
clock) core clocks are only present on RZ/G2UL, not on RZ/Five.

Annotate this in the comments, like is already done for module clocks
and resets.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/ffcdcd479c76b92f67481836a33ec86e97f85634.1708944903.git.geert+renesas@glider.be