dts: bindings: drop remaining "required: false" from bindings

Drop the remaining "required: false" from the bindings, make CI happy.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

dts/bindings/base/pm.yaml

@@ -5,7 +5,6 @@
 
 properties:
   wakeup-source:
-    required: false
     type: boolean
     description: |
       Property to identify that a device can be used as wake up source.
@@ -18,7 +17,6 @@ properties:
       the system) by default but they can be enabled at runtime if necessary.
 
   power-domain:
-    required: false
     type: phandle
     description: |
 

dts/bindings/base/power.yaml

@@ -6,7 +6,6 @@
 properties:
   supply-gpios:
     type: phandle-array
-    required: false
     description: |
       GPIO specifier that controls power to the device.
 
@@ -18,7 +17,6 @@ properties:
 
   vin-supply:
     type: phandle
-    required: false
     description: |
       Reference to the regulator that controls power to the device.
       The referenced devicetree node must have a regulator compatible.

dts/bindings/base/zephyr,memory-region.yaml

@@ -19,7 +19,6 @@ properties:
 
   zephyr,memory-region-mpu:
     type: string
-    required: false
     enum:
       - "RAM"
       - "RAM_NOCACHE"