rockchip: Kconfig: Enable SPL support for rk3568

Enable SPL support in Kconfig and add some related option in
rk3568_common.h

Signed-off-by: Nico Cheng <nico.cheng@rock-chips.com>
Signed-off-by: Jason Zhu <jason.zhu@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>