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>
diff --git a/doc/board/rockchip/rockchip.rst b/doc/board/rockchip/rockchip.rst
index fbb9983..8d75efc 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -26,6 +26,8 @@
      - Kylin (kylin_rk3036)
 * rk3128
      - Rockchip Evb-RK3128 (evb-rk3128)
+* rk3188
+     - Radxa Rock (rock)
 * rk3229
      - Rockchip Evb-RK3229 (evb-rk3229)
 * rk3288
@@ -75,8 +77,6 @@
 * rv1108
      - Rockchip Evb-rv1108 (evb-rv1108)
      - Elgin-R1 (elgin-rv1108)
-* rv3188
-     - Radxa Rock (rock)
 
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