doc: rockchip: sort rockchip support list for rk3188

In the list of mainline U-boot supported Rockchip boards
rk3188 is placed below under the name rv3188. Give back it's
original name and sort the list in alphabetical order.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>