drivers: thermal: Add a new DDR bus scaling cooling device

Add a new DDR cooling device which can request for a DDR state as a
part of cooling device request. This will help in the situation where
DDR has to be kept at a specific state during thermally constrained
situations.

Change-Id: I7acb6ee387dd21281b6a221591a9d1f98a3bc011
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>