drivers: imx: crash-console: Add a mxc_crash_console driver

drivers: imx: crash-console: Add a mxc_crash_console driver

This patch does two main things

- It implements the crash console UART init in assembly, as a
  hard-coded 115200 8N1 assumed from the 24 MHz clock.

  If the clock setup code has not run yet, this code can't work but,
  setting up clocks and clock-gates is way out of scope for this type of
  recovery function.

- It adds code to write a character out of the NXP UART without using any
  stack-based operations when doing so.

- Provides support for crash console in DCE or DTE mode.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>