Different Rockchip SoC variants have a different number of channels.
Introduce a channel mask to make the number of channels configurable
from SoC initialization code.

Link: https://lore.kernel.org/all/20231018061714.3553817-8-s.hauer@pengutronix.de/Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>