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 <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241010-bcm63138-leds-v4-1-cdb80780a555@linaro.org


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