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 <[e-mail]>
Reviewed-by: Daniel Thompson <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/leds/backlight/richtek,rt4831-backlight.yaml

···
     minimum: 0
     maximum: 3
 
+  richtek,bled-ocp-microamp:
+    description: |
+      Backlight over current protection level.
+    enum: [900000, 1200000, 1500000, 1800000]
+
   richtek,channel-use:
     description: |
       Backlight LED channel to be used.