leds: simatic-ipc-leds-gpio: Make sure we have the GPIO providing driver

If we register a "leds-gpio" platform device for GPIO pins that do not
exist we get a -EPROBE_DEFER and the probe will be tried again later.
If there is no driver to provide that pin we will poll forever and also
create a lot of log messages.

So check if that GPIO driver is configured, if so it will come up
eventually. If not, we exit our probe function early and do not even
bother registering the "leds-gpio". This method was chosen over "Kconfig
depends" since this way we can add support for more devices and GPIO
backends more easily without "depends":ing on all GPIO backends.

Fixes: a6c80bec ("leds: simatic-ipc-leds-gpio: Add GPIO version of Siemens driver")
Signed-off-by: Henning Schild <henning.schild@siemens.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221007153323.1326-1-henning.schild@siemens.com