zephyr: increase ec idle stack size for npcx9
zephyr: increase ec idle stack size for npcx9

Increase the idle thread stack to 400 bytes.
Without this change DUT reboots as stack overflows.

BUG=b:219891340
BRANCH=none
TEST=Random EC faults are not seen on Nivviks.

Change-Id: I5d367cc3626c9852b4e25589810981d969ba8834
Signed-off-by: Deepti Deshatty <f990118e5e566f6bd80b8c40bac496dfafb1a88f@intel.corp-partner.google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3492302
Reviewed-by: Vijay P Hiremath <006c712879a3af08bfdb5c2b8491a8b2d5fd73cd@intel.com>
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Commit-Queue: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

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