docs: system: arm: Introduce bananapi_m2u

Add documents for Banana Pi M2U

Signed-off-by: qianfan Zhao <qianfanguijin@163.com>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
[PMM: Minor format fixes to correct sphinx errors]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
2 files changed