board/storm : Add support for MMC GPIOs.

Pin assignment matches storm Proto0 schematics, drive strength was
discussed with the HW team and is appropriate for the current board
layout.

BUG=chrome-os-partner:27947

TEST=Tested MMC register reads and data reads, for example requesting
     for OCR, CID register, and switching the mode of operation.

Change-Id: Idea33fde3ca44a248716fb2aafcbd0db66f5ab5e
Signed-off-by: Yogesh Lal <ylal@codeaurora.org>
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/208987
Reviewed-by: Trevor Bourget <tbourget@codeaurora.org>