dt-bindings: backlight: rt4831: Add the new ocp level property

Add 'richtek,bled-ocp-microamp' property to make it chooseable.

The wrong backlight ocp level may affect the backlight channel output
current smaller than configured.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/1655807788-24511-2-git-send-email-u0084500@gmail.com