plankton: add flag for prog_en to flash_ec

Add setting of new GPIO, prog_en, to flash_ec to be able to program
new plankton boards. This pin must be on for boot0 and nrst to be
connected from the FTDI to the MCU.

BUG=chrome-os-partner:31633
BRANCH=none
TEST=manual,

sudo servod -c plankton.xml
util/flash_ec --board=plankton

CQ-DEPEND=CL:216160

Change-Id: I29f882856e24147a7af283c5e82298c7736b8662
Signed-off-by: Alec Berg <alecaberg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/216161
Reviewed-by: Todd Broch <tbroch@chromium.org>