dt-bindings: pwm: Add support for PWM nexus node

Platforms can have a standardized connector/expansion slot that exposes
PWMs signals to expansion boards.

A nexus node [1] allows to remap a phandle list in a consumer node
through a connector node in a generic way. With this remapping, the
consumer node needs to know only about the nexus node. Resources behind
the nexus node are decoupled by the nexus node itself.

This is particularly useful when this consumer is described in a
device-tree overlay. Indeed, to have the exact same overlay reused with
several base systems the overlay needs to known only about the connector
is going to be applied to without any knowledge of the SoC (or the
component providing the resource) available in the system.

As an example, suppose 3 PWMs connected to a connector. The connector
PWM 0 and 2 comes from the PWM 1 and 3 of the pwm-controller1. The
connector PWM 1 comes from the PWM 4 of the pwm-controller2. An
expansion device is connected to the connector and uses the connector
PMW 1.

Nexus node support in PWM allows the following description:
        soc {
                soc_pwm1: pwm-controller1 {
                        #pwm-cells = <3>;
                };

                soc_pwm2: pwm-controller2 {
                        #pwm-cells = <3>;
                };
        };

        connector: connector {
                #pwm-cells = <3>;
                pwm-map = <0 0 0 &soc_pwm1 1 0 0>,
                          <1 0 0 &soc_pwm2 4 0 0>,
                          <2 0 0 &soc_pwm1 3 0 0>;
                pwm-map-mask = <0xffffffff 0x0 0x0>;
                pwm-map-pass-thru = <0x0 0xffffffff 0xffffffff>;
        };

        expansion_device {
                pwms = <&connector 1 57000 0>;
        };

>From the expansion device point of view, the PWM requested is the PWM 1
available at the connector regardless of the exact PWM wired to this
connector PWM 1. Thanks to nexus node remapping described at connector
node, this PWM is the PWM 4 of the pwm-controller2.

[1] https://github.com/devicetree-org/devicetree-specification/blob/v0.4/source/chapter2-devicetree-basics.rst#nexus-nodes-and-specifier-mapping

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
[ukleinek: Make description node use folded style (>) as suggested by Rob]
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250205095547.536083-2-herve.codina@bootlin.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

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