realtek: add switch port LED driver

realtek: add switch port LED driver

RTL83xx switch SoCs can control up to 3 LEDs per (non-CPU) switch port,
typically to display the port link status. This driver allows to address
these LEDs and provides direct control from userspace, blink offloading,
and switch port status offloading. On Maple (RTL838x, RTL833x) 3×28 LEDs
are supported. On Cypress (RTL839x, RTL835x) 3×52 LEDs are supported.

The driver expects a list of devicetree nodes, one per physical LED, or
LED component in the case of multi-color LEDs. The 'reg' property of
these nodes has three cells:
  1. Switch's internal port number (0-27 for Maple, 0-51 for Cypress)
  2. Port LED index (0-2)
  3. Port type index (0 for RJ45/primary, 1 for SFP/secondary)

The driver refers to the RJ45 and SFP LEDs as "primary" and "secondary",
since SFP LED values are always output from the peripheral after RJ45
LED values. Note that on Maple it is not possible to control these
secondary LEDs from userspace. Nevertheless, if a combo port has one LED
per physical medium, both need to be specified in the devicetree.

Since offloading of the generic netdev trigger does not exist (yet),
this driver provides a private trigger which achieves the same and is
named "realtek-switchport". Each LED will have a file named
"rtl_hw_trigger" in /sys/class/leds/$LED, where the requested trigger
mode can be written to. It should be noted that port status offloading
is always enabled simultaneously for the primary and secondary LED.

When the custom trigger is activated, the LED will be added to a group
of LEDs with the same trigger conditions. If it is not possible to add
the LED to a group, e.g. when the number of control groups is exhausted,
the user must first choose an already existing trigger condition, or use
direct LED control.

If an unsupported mode is requested, or the trigger cannot be activated,
this will be reported to the user via a kernel message and write
failure.

Some common modes (i.e. values for "rtl_hw_trigger") are:
  - 1G/100M/10M Link present, blink on activity: 1f
  - 100M/10M link, blink on activity: f
  - 1G link present: 10
  - blink on link activity: 3

To document a port LED trigger as specified on the device's front panel,
the user can specify a u32 "realtek,port-led-trigger" property in the
DTS, using the helper macros from the dt-bindings header.

    /* Port 8, LED index 1, primary LED: Link/Act 1G LED */
    led@8.1.0 {
        reg = <8 1 0>;
        color = <LED_COLOR_ID_GREEN>;
        function = LED_FUNCTION_LAN
        linux,default-trigger = "realtek-switchport";
        realtek,port-led-trigger = <(RTL_LED_ACT | RTL_LED_LINK_1000)>;
    };

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Tested-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>