docs: Add skeletal documentation of cubieboard

Add skeletal documentation of the cubieboard machine.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210713142226.19155-2-peter.maydell@linaro.org

docs/system/arm/cubieboard.rst

@@ -0,0 +1,16 @@
+Cubietech Cubieboard (``cubieboard``)
+=====================================
+
+The ``cubieboard`` model emulates the Cubietech Cubieboard,
+which is a Cortex-A8 based single-board computer using
+the AllWinner A10 SoC.
+
+Emulated devices:
+
+- Timer
+- UART
+- RTC
+- EMAC
+- SDHCI
+- USB controller
+- SATA controller

docs/system/target-arm.rst

@@ -85,6 +85,7 @@ undocumented; you can get a complete list by running
    arm/aspeed
    arm/sabrelite
    arm/digic
+   arm/cubieboard
    arm/musicpal
    arm/gumstix
    arm/nrf