hw/i2c: pmbus: add fan support

hw/i2c: pmbus: add fan support

PMBus devices may integrate fans whose operation is configurable
over PMBus. This commit allows the driver to read and write the
fan control registers but does not model the operation of fans.

Reviewed-by: Stephen Longfield <slongfield@google.com>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Titus Rwantare <titusr@google.com>
Message-ID: <20231023-staging-pmbus-v3-v4-3-07a8cb7cd20a@google.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>