drivers: gpio: axp192: Add support to control EXTEN pin

AXP192 features an EXTEN pin that is on output only.

This commit appends control of EXTEN pin to gpio functionality
of AXP192.
Port-Mapping is as follows:
- [0..4]: GPIO0..4
- [5]: EXTEN

Signed-off-by: Martin Kiepfer <mrmarteng@teleschirm.org>

dts/bindings/gpio/x-powers,axp192-gpio.yaml

@@ -1,7 +1,18 @@
 # Copyright (c) 2023 Martin Kiepfer
 # SPDX-License-Identifier: Apache-2.0
 
-description: PXA192 GPIO Controller
+description: AXP192 GPIO Controller
+
+  AX192 features 5 native GPIOs. In addition the EXTEN pin can be configured
+  to be used as an IO as well.
+
+  Pin-Mapping is as follows
+    [0] GPIO0
+    [1] GPIO1
+    [2] GPIO2
+    [3] GPIO3
+    [4] GPIO4
+    [5] EXTEN
 
 compatible: "x-powers,axp192-gpio"
 
@@ -13,7 +24,7 @@ properties:
 
   ngpios:
     required: true
-    const: 5
+    const: 6
     description: |
       Number of GPIOs available on axp192.