Add chell_pd to the STM32 list and the USBPD override list.
BUG=chrome-os-partner:46289
BRANCH=none
TEST=successfully run "flash_ec --board=chell_pd"
Change-Id: Ic4ddbe51a0586c563211fd76f20a85428e565546
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/308726
Reviewed-by: Shawn N <shawnn@chromium.org>