snippets: NPCX 40 MHz support

Add a snippet for enabling 40 MHz operation on NPCX ECs. This snippet
includes both a devictree overlay and Kconfig override to properly
enable this feature.

BUG=none
TEST=zmake compare-builds pujjoga karis (with next CL)

Change-Id: I7bda57553500e56bca1e37c1d7ce4eb36903c81d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/7843234
Code-Coverage: Zoss <zoss-cl-coverage@prod.google.com>
Reviewed-by: Rob Barnes <robbarnes@google.com>
Commit-Queue: Keith Short <keithshort@chromium.org>
Reviewed-by: caveh jalali <caveh@chromium.org>
Tested-by: Keith Short <keithshort@chromium.org>