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>

doc/openocd.texi

@@ -595,6 +595,9 @@ produced, PDF schematics are easily found and it is easy to make.
 @item @b{bcm2835gpio}
 @* A BCM2835-based board (e.g. Raspberry Pi) using the GPIO pins of the expansion header.
 
+@item @b{imx_gpio}
+@* A NXP i.MX-based board (e.g. Wandboard) using the GPIO pins (should work on any i.MX processor).
+
 @item @b{jtag_vpi}
 @* A JTAG driver acting as a client for the JTAG VPI server interface.
 @* Link: @url{http://github.com/fjullien/jtag_vpi}
@@ -2992,6 +2995,18 @@ pinout.
 
 @end deffn
 
+@deffn {Interface Driver} {imx_gpio}
+i.MX SoC is present in many community boards. Wandboard is an example
+of the one which is most popular.
+
+This driver is mostly the same as bcm2835gpio.
+
+See @file{interface/imx-native.cfg} for a sample config and
+pinout.
+
+@end deffn
+
+
 @deffn {Interface Driver} {openjtag}
 OpenJTAG compatible USB adapter.
 This defines some driver-specific commands: