init: Optimize device_get_binding()

Improves the efficiency of the search by doing a single pass through
all device objects, rather than searching one level at a time.
Also enhances the comments for this routine.

Change-Id: I6786f24381f03d74d8c559ccbd7aa6a37683a8b7
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>