ueventd: track platform device uevents to parse block device names

The mmc block devices on panda boards use a platform device called
omap/omap_hsmmc.0.  The number of path entries in the uevent after
the platform device is unknown, and the number of path entries in
the platform device is unknown, so the only way to determine how
many entries make up the platform device is to compare against
a list of platform devices.

Add a list of platform devices that have been registered, and
iterate through it to determine the platform device portion of
a block device uevent.

Change-Id: I1480bc6429172ca6997d548787be3863ce062472