led_strip: ws2812_spi: Support configurable symbol width

This commit introduces a new devicetree property, bits-per-symbol, to
allow the symbol width to be configured from 3 to 8 bits. This change
is particularly beneficial for MCUs that lack DMA for their SPI
peripheral and have a limited hardware FIFO.

This property provides flexibility by allowing developers to select a
slower SPI clock frequency and use the symbol width to scale the
timings to meet strict LED strip requirements, minimizing the risk of
FIFO underruns.

Additionally, using higher-density patterns (e.g., 3-bit or 4-bit
symbols) makes more efficient use of the pixel buffer, which reduces
the RAM footprint required for the LED strip data.

The implementation is optimized with a fast path for the common 8-bit
symbol case, while a generic bit-packing loop handles all other
widths.

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