zephyr: shim: support optional pwm led channels
zephyr: shim: support optional pwm led channels

This changes the PWM_CHANNEL_BY_IDX macro to correctly handle pwm-leds
entries with less than 3 phandles, defaulting the corresponding channels
to PWM_LED_NO_CHANNEL.

This is necessary for devices with only two LED channels.

BRANCH=none
BUG=b:200974848
TEST=build only, compared the resulting pwm_leds structure

Signed-off-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>
Change-Id: I719ebfb085e60cc2ef27c3508c2b73ae85d3f7ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3205318
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

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