dt-binding: mfd: Add Richtek RT5120 PMIC support

Add Richtek RT5120 PMIC devicetree document.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/1660100142-32493-2-git-send-email-u0084500@gmail.com