serial: fsl-imx-uart: Cleanup duplicate device tree binding

We had two bindings for the same serial device, it looks like the one in
tty/serial/fsl-imx-uart.txt is the more up to date one so go with it and
merge a few things about the use/need for aliases in from
serial/fsl-imx-uart.txt.

Signed-off-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>