zephyr: zmake: add platform/ec/zephyr to DTS_ROOT
zephyr: zmake: add platform/ec/zephyr to DTS_ROOT

This allows us to put a dts directory and include directory in
platform/ec/zephyr for DTS preprocessing.

BUG=b:177003034
BRANCH=none
TEST=follow-up CLs

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I8999387cf948514d723f3dd8e876cabcd6bb6161
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2627955
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

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