arm: dts: rockchip: rk3066a-mk808: remove unused u-boot,boot-led

arm: dts: rockchip: rk3066a-mk808: remove unused u-boot,boot-led

There's no code to make use of it.

Additionally, if we ever want to enable this LED as Boot LED, we should
instead be using boot-led phandle property in /options/u-boot[1] Device
Tree node with the "new" LED UCLASS devices.

So let's simply remove this unused property to not mislead users.

[1] https://github.com/devicetree-org/dt-schema/blob/v2025.08/dtschema/schemas/options/u-boot.yaml#L113-L116

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>