drivers: gpio: smartbond: Fix PDC GPIO port selection

The GPIO block instance is based on the instance number during the
device driver initialization. This is not correct as instance numbers
in now way reflect any numbering scheme. Therefore, a DTS property
is introduced so that the block instance numbering is indicated
explicitly.

Signed-off-by: Ioannis Karachalios <ioannis.karachalios.px@renesas.com>