[bsp][stm32] 增加uart7和uart8的DMA配置
[bsp][stm32] 增加uart7和uart8的DMA配置

RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread