plankton: Set polarity when connected
plankton: Set polarity when connected

When a cable is connected, set USBC_POLARITY to the right polarity. This
is done in a different way than how we do this on other boards because
we only want to control polarity automatically on cable connection.

BUG=chrome-os-partner:32163
TEST=Flip the cable, check USBC_POLARITY changes.
BRANCH=None

Change-Id: I903123b8fd729e8c913014b83812d20328600f8e
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/219291
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>

akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,thehobn/ec,fourier49/BZ_DEV_EC,eatbyte/chromium-ec,thehobn/ec,mtk09422/chromiumos-platform-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,longsleep/ec,thehobn/ec,md5555/ec,mtk09422/chromiumos-platform-ec,fourier49/BZ_DEV_EC,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,coreboot/chrome-ec,mtk09422/chromiumos-platform-ec,md5555/ec,eatbyte/chromium-ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,longsleep/ec,fourier49/BIZ_EC,longsleep/ec,longsleep/ec,fourier49/BIZ_EC,eatbyte/chromium-ec,coreboot/chrome-ec,eatbyte/chromium-ec,thehobn/ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,fourier49/BZ_DEV_EC,md5555/ec,md5555/ec,coreboot/chrome-ec