led_strip: ws2812_spi: Optimize for 8-bit symbols with build-time check

The ws2812_spi driver supports both a fast path for 8-bit symbols and
a generic path for other sizes.

This change introduces a devicetree-based, build-time check to
determine if all enabled instances use 8-bit symbols. If so, the
generic path logic is removed from the build.

Signed-off-by: Wai-Hong Tam <waihong@google.com>