drivers: of: Add APIs to find DDR device rank, HBB

Add APIs to find the DDR device rank and Highest
Bank Bit (HBB) information from their respective
memory nodes.

Change-Id: Iae94efc681daf2cf0a531dceb8d2acdcf3484388
Signed-off-by: Kaushal Kumar <kaushalk@codeaurora.org>