spmi: Remove redundant controller list

Registered controllers are maintained in a linked list as well as in an
idr (ID management) object which is redundant.  This patch removes the
linked list and just uses the idr to manage the controllers.

Change-Id: Ief36e6870b036304e34d4eec4e22e50c6c028d43
Signed-off-by: Kenneth Heitke <kheitke@codeaurora.org>