audio.example.xml: Document finding card by name
audio.example.xml: Document finding card by name

Document that the <mixer> element can take either a card attribute
or a name attribute.

Change-Id: I737375a0909934f91f1088640908dfaf475ff430
Signed-off-by: Richard Fitzgerald <22ee0097135072451429b5ac45630d52779cd49b@opensource.cirrus.com>

CirrusLogic/tinyhal,CirrusLogic/tinyhal,CirrusLogic/tinyhal