serial_mxc: disable new features of autobaud detection

Bit 7 of UCR3 is described in the i.MX3x/i.MX5x/i.MX6x
reference manuals as follows:

	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

On at least i.MX6DQ, i.MX6DLS and i.MX53, the "new features"
occasionally cause the receiver to get out of sync and
continuously produce received characters of '\xff'.

This patch disables the "new feature" on all boards, since
there's no support for auto-baud in U-Boot on any of them.

More details are available in this post on i.MX Community:
	https://community.freescale.com/message/403254



Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>