kukui: Disable eMMC power supply on boot
kukui: Disable eMMC power supply on boot

Allows reworked boards to boot using eMMC emulation, without
manual steps.

BRANCH=none
BUG=b:111773571
TEST=Flash and boot kukui

Change-Id: I0a28bcd5a047f708996ee6ac214968e269e17ffe
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1149426
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Reviewed-by: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec