drivers: gpio: shell: improve tab complete/suggestion support

This commit implements this enhancement:
https://github.com/zephyrproject-rtos/zephyr/issues/63018



The forms of the gpio commands are now:

        gpio conf device pin ol0
        gpio set device pin 1
        gpio get device pin
        gpio blink device pin

Device name and pin subcommands now are
suggested/completed when tab is used.

Pin names are suggested with numbers and line names if
available from the gpio controller’s Devicetree node.

GPIO pin command is now limited to pins that are not assigned
as reserved.

Signed-off-by: Nick Ward <nix.ward@gmail.com>