Convert the da9062 PMIC device tree binding documentation to json-schema.

Document the missing gpio child node for da9062.

While at it, update description with link to product information and
example.

The missing child node with of_compatible defined in MFD_CELL_OF is
causing the below warning message:
da9062-gpio: Failed to locate of_node [id: -1]

So, make all child nodes with of_compatible defined in struct mfd_cell
as required property for da906{1,2} devices.

The "gpio-controller" and "#gpio-cells" properties are defined in the
parent instead of gpio child node as there are existing driver users
based on these parent properties.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20240131102656.3379-7-biju.das.jz@bp.renesas.comSigned-off-by: Lee Jones <lee@kernel.org>