zephyr: add GPIO enum GPIO_USB_CX_DISHCARGE
zephyr: add GPIO enum GPIO_USB_CX_DISHCARGE

It's missing so add the enum.

BUG=none
TEST=zmake testall
BRANCH=none

Change-Id: Ie3ac609a29778316c6c11252cb5d57297525bcca
Signed-off-by: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3512277
Reviewed-by: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>
Commit-Queue: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@google.com>
Tested-by: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@google.com>

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