drivers: gpio: use I2C_INIT_PRIORITY as default for gpio chips on i2c bus

use I2C_INIT_PRIORITY as default for gpio chips,
that are on a i2c bus.

If childs have the same priority as their parents,
the init order is decided by the devicetree ordinals.
This ensures, that these childs are init after their parent.
Because of that gpio chips on a i2c bus can have the
same priority as the i2c controller.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>