ASoC: rsnd: remove platform boot support from src.c

No board is using Renesas sound driver via platform boot now.
This means all user is using DT boot. Platform boot support is
no longer needed. But, it strongly depends on platform boot style.
This patch removes platform boot support from src.c

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>