drivers/imx_gpio: fix usage messages

drivers/imx_gpio: fix usage messages

The notation "(tck tms tdi tdo)* " is incorrect, because it means the
quadruple of gpio can be repeated on the command-line.
The correct syntax of the command requires instead to provide either
all the four gpio numbers (in order to set the values) or to pass an
empty command-line (to dump the values previously set).

Change the .usage field to "[tck tms tdi tdo]".
Change similarly the corresponding .usage field for SWD command.
Add the .usage field for the commands that individually set each gpio
or gpio property.
Dump the previously set values when commands imx_gpio_speed_coeffs or
imx_gpio_peripheral_base are executed with empty command-line.

Change-Id: Id56a68622817101bd0a46fed1fead6d2cd2b6ec6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5018
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>