hwmon: (pmbus) Implement multi-phase support

hwmon: (pmbus) Implement multi-phase support

Some PMBus chips support multiple phases, and report telemetry such
as input current, output current, or temperature for each phase.
Add support for such chips to the PMBus core.

Start with a maximum of 8 phases per page, and assume that supported
sensors per phase are similar for all pages. Only support per-phase
telemetry attributes, no limits or alarms.

As part of this patch, set the initial page variable to 0xff to ensure
that the page is updated when the first page command is issued. Also
only issue page commands if the chip supports more than one page.

Cc: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>