devicetree: bindings: Document micrel vendor prefix

There's one existing use of 'micrel' in the documentation so use
'micrel' instead of the company's ticker symbol 'mcrl'.

Cc: <devicetree@vger.kernel.org>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -75,6 +75,7 @@ linux	Linux-specific binding
 lsi	LSI Corp. (LSI Logic)
 marvell	Marvell Technology Group Ltd.
 maxim	Maxim Integrated Products
+micrel	Micrel Inc.
 microchip	Microchip Technology Inc.
 mosaixtech	Mosaix Technologies, Inc.
 moxa	Moxa