fixed-regulator: dt-bindings: Fixed building error for compatible property

Compatible property is not of type 'string', so remove const:
from it.

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1568875145-2864-1-git-send-email-pragnesh.patel@sifive.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/fixed-regulator.yaml

@@ -30,8 +30,8 @@ if:
 properties:
   compatible:
     enum:
-      - const: regulator-fixed
-      - const: regulator-fixed-clock
+      - regulator-fixed
+      - regulator-fixed-clock
 
   regulator-name: true