imx_gpio: add mmap based jtag interface for IMX processors

For some targets (like nrf51) sysfs driver is too slow. This
patch implements memory maped driver for IMX processors.
Mostly based on bcm2835gpio. Tested on imx6ul CPU. However, it should
work on any NXP IMX CPU.

Change-Id: Idace4c98181c6e9c64dd158bfa52631204b5c4a7
Signed-off-by: Grzegorz Kostka <kostka.grzegorz@gmail.com>
Reviewed-on: http://openocd.zylin.com/4106


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>