rockchip: Add mention of other boards

At present some Rockchip SoCs and boards are not mentioned in the README.
So that people can see which SoCs are supported, expand the list to
include everything.

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>