rockchip: rk3188: remove setup_led from xPL

rockchip: rk3188: remove setup_led from xPL

There's not a single device making use of that code and it anyway
shouldn't be using the old deprecated u-boot,boot-led /config property
anymore but rather boot-led from /options/u-boot[1] Device Tree node.

Because spl_board_init() is only present to call this now removed
function, we can remove it as well as SPL_BOARD_INIT which is the symbol
guarding calls to spl_board_init() (which is now also removed).

[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>