CHROMIUM: cros_ec_pd_update: Add polarity discovery.

Add ability to discover the USB type-C polarity (normal or flipped)
via the current EC driver.

Presently this is a exported to be used directly in the DisplayPort
driver.  That should be undone once better solution, crbug.com/488161,
is in place.

BUG=chrome-os-partner:35351,chrome-os-partner:40076
BUG=b:123092035
TEST=compiles

Change-Id: I244f0e5b700fb022056a51e55b3663f90b32739c
Signed-off-by: Todd Broch <tbroch@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/270333


Reviewed-by: Stphane Marchesin <marcheu@chromium.org>
[jflat: cleaned up cros_ec_pd_update_check]
Signed-off-by: Jon Flatley <jflat@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1423678


Reviewed-by: Dmitry Torokhov <dtor@chromium.org>