dt-bindings: mfd: Convert lp873x.txt to json-schema

Convert the lp873x binding to DT schema format. The gpio-controller
and #gpio-cells properties were removed from required because using
the device as a GPIO controller is optional.

Signed-off-by: Nathan Morrisson <nmorrisson@phytec.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240429195830.4027250-1-nmorrisson@phytec.com


Signed-off-by: Lee Jones <lee@kernel.org>