tty: serial: qcom-geni-serial: add support for serial engine DMA

The qcom-geni-serial driver currently only works in SE FIFO mode. This
limits the UART speed to around 180 kB/s. In order to achieve higher
speeds we need to use SE DMA mode.

Keep the console port working in FIFO mode but extend the code to use DMA
for the high-speed port.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>