hwmon: (pmbus) Add IC_DEVICE_ID and IC_DEVICE_REV command definitions

Recent PMBus versions added IC_DEVICE_ID and IC_DEVICE_REV commands as
additional means to identify the chip. Add command definitions to
pmbus.h include file.

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