Chell: Enable eMMC HS400 mode With CL317451 and CL317267, Hynix eMMC can run under HS400 mode. BUG=chrome-os-partner:47647 TEST=run consective boot 100 times on Chell EVT Hynix SKU, and MMC errors didn't happen. BRANCH=none Change-Id: I6bec88f5c2813131a693ddba5523a9d43b2ebd45 Signed-off-by: Ryan Lin <[email protected]> Reviewed-on: https://chromium-review.googlesource.com/319627 Commit-Ready: Duncan Laurie <[email protected]> Tested-by: Duncan Laurie <[email protected]> Reviewed-by: Duncan Laurie <[email protected]>