serial: imx: Disable new features of autobaud detection

Bit 7 of UCR3 is described in the i.MX reference manuals (with the exception
of i.MX1) as follows:

ADNIMP: Autobaud Detection Not Improved-. Disables new features of
	autobaud detection (See Baud Rate Automatic Detection
	Protocol, for more details).

	0 Autobaud detection new features selected
	1 Keep old autobaud detection mechanism

The "new features" mechanism occasionally cause the receiver to get out of sync
and continuously produce received characters of '\xff'.

In order to reproduce the problem:

$ stty -F /dev/ttymxc0 19200

- Change the terminal baudrate to 19200

- Type in the console and it should look good

- Change the terminal baudrate back to 115200

- Type 'b' in the console, then a stream of garbage characters is seen.

Also rename the bit definition as per the reference manual.

Tested on mx6q, mx6dl, mx6solo and mx53.

Based on a patch from Eric Nelson for U-boot.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>