rcar: src: skip disabled-SRC nodes

The current device tree representation of the R-Car Sample Rate Converters
(SRC) assumes that they are numbered consecutively, starting from 0. Alas,
this  is not  the case with the R8A7794 SoC where SRC0 isn't present.  In
order to keep the existing  device trees working, I'm suggesting to use a
disabled node for SRC0.  Teach the SRC probe  to just skip disabled nodes.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>