ASoC: rsnd: rename scu to src

R-Car sound has SCU unit which has SRC/CTU/MIX/DVC,
and current rsnd driver has scu.c and scu module.
Current scu.c has SRC support only.
My first concept was control these feature on scu.c
but, it become difficult and un-understandable now.
This patch rename scu to src

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