extend agent to read gpio stub

[guycole]

extend agent to return gpio stub, like

root@beaglebone:~# gpiodetect
gpiochip0 [gpio-0-31] (32 lines)
gpiochip1 [gpio-32-63] (32 lines)
gpiochip2 [gpio-64-95] (32 lines)
gpiochip3 [gpio-96-127] (32 lines)

gsc@rpi4e:~ $ gpiodetect
gpiochip0 [pinctrl-bcm2711] (58 lines)
gpiochip1 [raspberrypi-exp-gpio] (8 lines)
gpiochip0 [pinctrl-bcm2711] (58 lines)

should suppress duplicates (see rpi returns gpiochip0 twice)