dt-bindings: serial: renesas,rsci: Add optional secondary clock input

dt-bindings: serial: renesas,rsci: Add optional secondary clock input

Update the RSCI binding to support an optional secondary clock input on
the RZ/T2H SoC. At boot, the RSCI operates using the default synchronous
clock (PCLKM core clock), which is enabled by the bootloader. However, to
support a wider range of baud rates, the hardware also requires an
asynchronous external clock input. Clock selection is controlled
internally by the CCR3 register in the RSCI block.

Due to an incomplete understanding of the hardware, the original binding
defined only a single clock ("fck"), which is insufficient to describe the
full capabilities of the RSCI on RZ/T2H. This update corrects the binding
by allowing up to three clocks and defining the `clock-names` as
"operation", "bus", and optionally "sck" for the asynchronous clock input.

This is an ABI change, as it modifies the expected number and names of
clocks. However, since there are no in-kernel consumers of this binding
yet, the change is considered safe and non-disruptive.

Also remove the unneeded `serial0` alias from the DTS example and use
the R9A09G077_CLK_PCLKM macro for core clock.

Signed-off-by: Thierry Bultel <thierry.bultel.yh@bp.renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: "Rob Herring (Arm)" <robh@kernel.org>
Link: https://lore.kernel.org/r/20250630202323.279809-2-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>