Define the generic r8a7743(RZ/G1M) part of  the sound device node.

This patch is based on the r8a7791 sound work by Kuninori Morimoto.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>