dt-bindings: timer: renesas,rz-mtu3: Use #pwm-cells = <3>

dt-bindings: timer: renesas,rz-mtu3: Use #pwm-cells = <3>

With the goal to unify all PWM bindings to use #pwm-cells = <3> update
the renesas,rz-mtu3 binding accordingly. Keep <2> documented as a
deprecated value at least until the in-tree device trees are fixed
accordingly.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20250527205823.377785-2-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>