Add documentation of new properties for sample averaging in PMON_CONFIG
register.

New properties:
- adi,volt-curr-sample-average
- adi,power-sample-average
Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220302123817.27025-3-potin.lai@quantatw.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>