dt-bindings: vendor-prefixes: Add yet another for ST-Ericsson

Unfortunately the vendor prefix for ST-Ericsson is used very
inconsistently. "ste," and "stericsson," are already documented,
but some things in the kernel use "st-ericsson," which is not
documented yet.

st-ericsson,u8500 is documented in bindings/arm/ux500/boards.txt,
and is used to match the machine code and the generic DT cpufreq
driver.

Add it to the list of vendor prefixes.
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.yaml

@@ -913,6 +913,8 @@ patternProperties:
     description: ST-Ericsson
   "^stericsson,.*":
     description: ST-Ericsson
+  "^st-ericsson,.*":
+    description: ST-Ericsson
   "^summit,.*":
     description: Summit microelectronics
   "^sunchip,.*":