dma: rcar-audmapp: don't keep audmapp_slave_config for each channeles

Current audmapp driver is keeping audmapp_slave_config
for each channeles, but, nessasary information is only "chcr".
Current style (= keeping audmapp_slave_config) is
not good match for DT support.
Keep "chcr" instead of audmapp_slave_config

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>