MXC: Add AUDMUXv2 register decode to debugfs

Since AUDMUX configuration appears to be one of the common stumbling
blocks for people setting up i.MX audio try to provide some diagnostic
information describing the current setup to assisist people in working
out what's going on.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>