leds: bcm63138: Handle shift register config

leds: bcm63138: Handle shift register config

This adds code to optionally read the width of the shift register
chain from the device tree and use it to set up the register
controlling the shifter hardware.

If the property is not present, the boot-time default is used so
existing device trees keep working as this is what they assume.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20241010-bcm63138-leds-v4-3-cdb80780a555@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>