zephyr: adc: Add an empty enum when not ADC is present
zephyr: adc: Add an empty enum when not ADC is present

At present we get warnings when adc.h is included but there is no
ADC available, such as with posix-ec. Correct this by adding an empty
enum in this case.

BUG=none
BRANCH=none
TEST=CQ

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: I139c48d0de860657036d071e596dea426225b52d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2842205
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>
Commit-Queue: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>

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