board: storage: enable eMMC DDR capability for non-CDP platforms

When eMMC runs in DDR mode on CDP platform, we have seen instability
due to DATA CRC errors. These errors are attributed to long physical
path between MSM and eMMC on CDP.

So disable the DDR mode on CDP platform only and let other platforms
take advantage of eMMC DDR mode.

NOTE: eMMC DDR mode instability was observed only on MSM8960 and
MSM8930 CDP platforms.

CRs-Fixed: 371242
Change-Id: I882e7a7c3e352a0f9c2ebe4362c55c22decef53b
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>