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: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rob Herring <robh@kernel.org>