dt-bindings: gnss: u-blox: add safeboot gpio

U-Blox M8/M9 chip have a pin to start it in safeboot mode, to be used
to recover from situations where the flash content has become
corrupted and needs to be restored.

Signed-off-by: Alejandro Enrique <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>

Documentation/devicetree/bindings/gnss/u-blox,neo-6m.yaml

···
   reset-gpios:
     maxItems: 1
 
+  safeboot-gpios:
+    maxItems: 1
+
   vcc-supply:
     description: >
       Main voltage regulator
···
             v-bckp-supply = <&gnss_v_bckp_reg>;
             vcc-supply = <&gnss_vcc_reg>;
             reset-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
+            safeboot-gpios = <&gpio 2 GPIO_ACTIVE_LOW>;
         };
     };