dt-bindings: mfd: rohm,bd71847-pmic: Correct clock properties requirements

The input clock and number of clock provider cells are not required for
the PMIC to operate.  They are needed only for the optional bd718x7
clock driver.

Add also clock-output-names as driver takes use of it.

This fixes dtbs_check warnings like:

    arch/arm64/boot/dts/freescale/imx8mn-ddr4-evk.dt.yaml: pmic@4b: 'clocks' is a required property
    arch/arm64/boot/dts/freescale/imx8mn-ddr4-evk.dt.yaml: pmic@4b: '#clock-cells' is a required property

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Rob Herring <robh@kernel.org>