perf(imx): speed-up console/uart TX using FIFO

perf(imx): speed-up console/uart TX using FIFO

The current putc version test for TXEMPTY bit set (#6) instead
of waiting for TXFULL bit clear (#4), that slows the global
boot time as we are not taking benefit of the 32-byte FIFO.

We then need to implement the flush function to be sure the
transmit is complete (FIFO and shift register empty).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Change-Id: I54873a5203e2afdc230e44ce73284e7a80985b4f