OMAP: hwmod: separate list locking and hwmod hardware locking

Currently omap_hwmod_mutex is being used to protect both the list
access/modification and concurrent access to hwmod functions.  This
patch separates these two types of locking.

First, omap_hwmod_mutex is used only to protect access and
modification of omap_hwmod_list.  Also cleaned up some comments
referring to this mutex that are no longer needed.

Then, for protecting concurrent access to hwmod functions, use a
per-hwmod mutex.  This protects concurrent access to a single hwmod,
but would allow concurrent access to different hwmods.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
[paul@pwsan.com: added structure documentation; changed mutex variable
 name]
Signed-off-by: Paul Walmsley <paul@pwsan.com>