valleyisland-io: Force BYT SDCARD host to run with SDR25

SD Card v3.01 in the market is not compatible with BYT SDHC IP
in SoC because this IP only support SD v3.0 only. So, we are
forcing DDR50 mode to always step down to SDR25.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>