drivers/bcm2835gpio: 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 bcm2835gpio_speed_coeffs
or bcm2835gpio_peripheral_base are executed with empty command-line.

Change-Id: Ie45d8268c1de331aded0bb52d5e6b8f0e8766a5b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5019


Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins