STM32F7: add warning for RXDMA + IFLOWCONTROL combination

Combination of RXDMA + IFLOWCONTROL does not work as one might expect.
Since RXDMA uses circular DMA-buffer, DMA will always keep reading new
data from USART peripheral even if DMA buffer underruns. Thus this
combination only does following: RTS is asserted on USART setup and
deasserted on shutdown and does not perform actual RTS flow-control.

Data loss can be demonstrated by doing long up_mdelay inside irq
critical section and feeding data to RXDMA+IFLOWCONTROL UART.