drivers: of: Add API to find ddr device type

Add API to find the ddr device type from memory node.

Change-Id: I1cfc38d46f1ea0abc6fbe8cbb6e37cde72b9fc2e
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>
[eberman@codeaurora.org: Move func to header]
Signed-off-by: Elliot Berman <eberman@codeaurora.org>