board: rockchip: add Powkiddy X55

The Powkiddy X55 is a Rockchip RK3566 based handheld gaming device.
UART, ADC, eMMC, and SDMMC are tested to work in U-Boot and this
successfully boots mainline Linux.

Kernel commit:
e99adc97e21a ("arm64: dts: rockchip: Add Powkiddy X55")

Signed-off-by: Chris Morgan <macromorgan@hotmail.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 9a726e9..924a8d0 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -104,6 +104,7 @@
      - Pine64 SOQuartz on Blade (soquartz-blade-rk3566)
      - Pine64 SOQuartz on CM4-IO (soquartz-cm4-rk3566)
      - Pine64 SOQuartz on Model A (soquartz-model-a-rk3566)
+     - Powkiddy X55 (powkiddy-x55-rk3566)
      - Radxa CM3 IO Board (radxa-cm3-io-rk3566)
 
 * rk3568