dt-bindings: iio: proximity: hx9023s: support firmware-name property

dt-bindings: iio: proximity: hx9023s: support firmware-name property

The hx9023s requires a firmware blob containing board-specific
configuration data used to initialize its internal sensing engine.

Although the silicon is identical across platforms, different products
may use different electrode layouts, PCB routing, cover materials and
mechanical stack-ups. These physical differences require distinct
calibration parameters and register configuration tables in order for
the sensor to operate correctly.

The driver has always required firmware and historically assumed a
single default firmware file name suitable for the reference design.
However, this assumption does not hold for boards with different
physical sensor layouts.

The default firmware file name remains unchanged and continues to be
used for existing platforms. Allowing the firmware file name to be
specified via device tree enables selecting the appropriate
hardware-specific configuration when the board design differs.

This property does not change the existing ABI and is optional.

Signed-off-by: Yasin Lee <yasin.lee.x@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>