dt-bindings: leds: bcm63138: Add shift register bits

The BCM63138 family of serial LED controllers has a register
where we can set up bits for the shift registers. These are
the number of rounds the bits need to be shifted before all
bits have been shifted through the external shift registers.

Signed-off-by: Linus Walleij <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/leds/leds-bcm63138.yaml

···
   "#size-cells":
     const: 0
 
+  brcm,serial-shift-bits:
+    minimum: 1
+    maximum: 32
+    description:
+      This describes the number of 8-bit serial shifters connected to the LED
+      controller block. The hardware is typically using 8-bit shift registers
+      with 8 LEDs per shift register, so 4 shifters results in 32 LEDs or 2
+      shifters give 16 LEDs etc, but the hardware supports any odd number of
+      registers. If left unspecified, the hardware boot-time default is used.
+
 patternProperties:
   "^led@[a-f0-9]+$":
     type: object
···
     leds@ff800800 {
         compatible = "brcm,bcm4908-leds", "brcm,bcm63138-leds";
         reg = <0xff800800 0xdc>;
+        brcm,serial-shift-bits = <16>;
 
         #address-cells = <1>;
         #size-cells = <0>;