dt-bindings: mfd: Add Rockchip rk817 battery charger support

Create dt-binding documentation to document rk817 battery and charger
usage. New device-tree properties have been added.

- rockchip,resistor-sense-micro-ohms: The value in microohms of the
                                      sample resistor.
- rockchip,sleep-enter-current-microamp: The value in microamps of the
                                         sleep enter current.
- rockchip,sleep-filter-current: The value in microamps of the sleep
                                 filter current.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Signed-off-by: Maya Matuszczyk <maccraft123mc@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220827021623.23829-2-macroalpha82@gmail.com