ALSA: seq: Rewrite sequencer device binding with standard bus

We've used the old house-made code for binding the sequencer device
and driver.  This can be far better implemented with the standard
bus nowadays.

This patch refactors the whole sequencer binding code with the bus
/sys/bus/snd_seq.  The devices appear as id-card-device on this bus
and are bound with the drivers corresponding to the given id like the
former implementation.  The module autoload is also kept like before.

There is no change in API functions by this patch, and almost all
transitions are kept inside seq_device.c.  The proc file output will
change slightly but kept compatible as much as possible.

Further integration works will follow in later patches.

Signed-off-by: Takashi Iwai <tiwai@suse.de>