hwmon: (pmbus) Introduce PMBUS_VIRT_*_SAMPLES registers

Those virtual registers can be used to export manufacturer specific
functionality for controlling the number of samples for average values
reported by the device.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>