soc: altmode: Update client APIs, notifier bookkeeping

Client API changes:
- Update client APIs to align them more closely with upstream
  conventions.
- In particular, change the link between altmode device and client from
  the altmode device's name string to a phandle to the altmode device
  followed by the port index of the client.
- Instead of using solely the client's SVID as the key for idr_alloc,
  include port_index as well because the unique identifier of a client
  is the (SVID, port_index) tuple.

Updates to client notifying mechanism:
- Get rid of the standard notifier APIs in favour of a simpler callback
  function-based mechanism to notify clients of probe completion.
- In addition, if clients probe after altmode finishes probing, they
  will not get notified of probe completion as this is done only once at
  the end of altmode_probe(). Fix this so that if a client registers for
  probe completion notifications after altmode probes, they get notified
  immediately.

While at it, get rid of the global list of altmode devices (amdev_list)
as there will only be a single altmode device in the system as per
design.

Change-Id: Iea450d30d9cc94671f505b7ff0c6e98b0b8a4467
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>