media: atomisp: place all gpio parsing together

Instead of parsing GPIO for two exceptions inside the logic
which would be trying to use the GPIO, move the init code
to the routine which adds a new gmin device.

Move the notes to it too, as this helps to identify where
those two GPIO settings are used, explaining why they're
defaulting to -1 when not found.

Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>