[PATCH v3 0/2] pwm: Add support for pwm nexus node

[Herve Codina <herve.codina@xxxxxxxxxxx>]

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>;
        };