zephyr: increase the number of preemptive threads
zephyr: increase the number of preemptive threads

The Byra platform has a total of 17 threads. Increase the maximum number
of preemptive threads to 20. There is no flash or RAM impact to setting
CONFIG_NUM_PREEMPT_PRIORITIES above the minimum required.

BUG=none
BRANCH=none
TEST=zmake testall

Signed-off-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Change-Id: I16078b668d28d45e5a19ce2f81d0fad8ba1fe466
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3562039
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@google.com>

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