pmbus: add PMBus 1.x framework, CLI, and binding

Add U-Boot's PMBus 1.x layer: the decoder/transport library, the
pmbus CLI command and a generic DT binding.

The subsequent commits provide the UCLASS_REGULATOR adapter and per-chip
drivers.

U-Boot's PMBus support is not a hwmon clone of Linux's
drivers/hwmon/pmbus/. Linux owns the runtime side (polling, sysfs,
alert IRQs, fan loops). U-Boot owns the boot-time side in order to,

 - identify the PMBus regulators a board carries: MFR_ID/
   MFR_MODEL/MFR_REVISION + sanity checks.
 - print telemetry (VIN/VOUT/IIN/IOUT/POUT/TEMP) so an
   operator can confirm rail voltages and faults before the kernel
 - decode any chip alerts (STATUS_VOUT/STATUS_IOUT/STATUS_INPUT/
   STATUS_TEMPERATURE/STATUS_CML) so a boot log shows why the
   previous boot failed or the board had been power cycled because
   of an outage (typically over temperature or under current).

Out of scope by design: no periodic polling, no sysfs, no fan-speed
control loop, no PMBUS_VIRT_* sensor virtualisation, no caching.
If a use case needs any of those, the answer should be "wait until
Linux comes up". It shall remain a thin layer.

The constants and structural shape (command codes, status bit names,
sensor-class enum, format enum, struct pmbus_driver_info) are
mirrored from Linux drivers/hwmon/pmbus/pmbus.h verbatim. The
decoders/encoders are reimplemented from the PMBus 1.3
specification because the surrounding hwmon context (struct
pmbus_data, sysfs caching, hwmon publication) does not apply.

The main benefits:

  - One framework + CLI for any board carrying PMBus regulators:
    no per-board PMBus implementation required anymore.
  - Boards call pmbus_print_telemetry() / pmbus_print_status_word()
    directly from boot init for a snapshot, sharing all decode +
    format-dispatch with the CLI.
  - Linux-compatible constants and DT binding so porting an existing
    drivers/hwmon/pmbus/ chip is mechanical.
  - Boot-time AVS/VID rail trim reuses the same decoders and
    encoders as the CLI and the regulator path: no duplicate math.

Signed-off-by: Vincent Jardin <[email protected]>
Signed-off-by: Peng Fan <[email protected]>

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