drivers/sysfsgpio: fix usage messages

drivers/sysfsgpio: 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.

Change-Id: If5b3c618097b71dfe7fcf988fb3c1499ae03a6d5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4833
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>