samples: drivers: led_ws2812: use compatible based filter

Replace 'platform_allow' with 'filter' based on ws2812 compatible
being enabled. This will allow, for example, to run the sample on
mimxrt1050_evk and mimxrt1050_evk_qspi boards.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>