dt-bindings: Add vendor prefix for Espressif

Add Espressif Systems DT vendor prefix. That prefix has been used for quite
some time for WiFi chips, but has never been documented.

Signed-off-by: Maxime Ripard <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/vendor-prefixes.yaml

···
     description: Ecole Polytechnique Fédérale de Lausanne
   "^epson,.*":
     description: Seiko Epson Corp.
+  "^esp,.*":
+    description: Espressif Systems Co. Ltd.
   "^est,.*":
     description: ESTeem Wireless Modems
   "^ettus,.*":