zephyr: shim: add an example for the leds property and make it required
zephyr: shim: add an example for the leds property and make it required

Add a complete example for the cros-ec,pwm-leds leds property, and mark
it as required since the whole LED functionality depends on it.

BRANCH=none
BUG=none
TEST=build and run

Signed-off-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>
Change-Id: I6d875b1a3a436baa77b6c15b8018e4b37102bb46
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3174051
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