soundwire: add new function to enumerate slave devices

Add a new function call in soundwire framework to
perform any remaining initialization of soundwire
slave devices. This function should be called right
after registering all slave devices with master,
and can perform a number of operations for enumerating
and registering the slave device with appropriate
subsystem (ALSA in this case).

Change-Id: Ie0a2f031eebdf5861eeef489bd94efb3682a0910
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>