imx: add polled io uart methods

These methods are invoked if the iMX uart is used in conjuction with kgdb during
early boot.  In order to access the UART without the interrupts, the kernel uses
the basic polling methods for IO with the device.  With these methods
implemented, it is now possible to enable kgdb during early boot over serial.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>
Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
CC: Sascha Hauer <s.hauer@pengutronix.de>
CC: Fabio Estevam <festevam@gmail.com>
CC: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
CC: linux-serial@vger.kernel.org
CC: Alan Cox <alan@linux.intel.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>