samples: drivers: led_ws2812: Rename boards files to follow hwmv2

The esp32s3_devkitm and the nrf5340dk_nrf5340 only have ws2812 in
each configurations that is procpu and cpuapp respectively.
Rename overlay and conf files to avoid building with unnecessary
configurations.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>