rockchip: Drop note about supporting other SoCs

Quite a wide range of Rockchip SoCs are supported in mainline U-Boot now,
so drop the comment about needing to add more.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>