Enable DMA on usart3 to get a more reliable console. This is especially
useful for automation and kernelci were a kernel with PROVE_LOCKING enabled
is quite susceptible to character loss, resulting in tests failure.

Cc: stable <stable@vger.kernel.org> #v4.1+
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>