asoc: Add sysfs attr for SPDIF interfaces

The format change event signaling for SPDIF input interfaces
is adapted to provide format information through sysfs nodes.

Change-Id: Ifab1074a0e4b13e08813e27f8694023c2c47bfe1
Signed-off-by: Ralf Herz <rherz@codeaurora.org>
Signed-off-by: Mangesh Kunchamwar <mangeshk@codeaurora.org>