platform/chrome: cros_kbd_led_backlight: support EC PWM backend

EC PWM backend uses EC_CMD_PWM_SET_KEYBOARD_BACKLIGHT and
EC_CMD_PWM_GET_KEYBOARD_BACKLIGHT for setting and getting the brightness
respectively.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Tested-by: Matthias Kaehlcke <mka@chromium.org>
Link: https://lore.kernel.org/r/20220523090822.3035189-6-tzungbi@kernel.org