servo_v4: disable PD entirely when in SNK.
servo_v4: disable PD entirely when in SNK.

This prevents the system from negotiating power or reenabling
dualrole after it's turned off in board.c.

BUG=b:129882930
BRANCH=servo
TEST=cc snkdts stays snk.

Change-Id: I0a05eb9e8073cec7da884e071e023bcddadb130f
Signed-off-by: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1553308
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Reviewed-by: Ruben Rodriguez Buchillon <12b0357cd989b603267f0b7cb81c747471f94bd1@chromium.org>

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