rockchip: ringneck-px30: enable SPL_BOARD_INIT

Now that Ringneck requires some board-specific code (namely resetting
the MCU companion controller) to be run during SPL stage, let's enable
SPL_BOARD_INIT.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>