PM / devfreq: rockchip-dfi: give variable a better name

PM / devfreq: rockchip-dfi: give variable a better name

struct dmc_count_channel::total counts the clock cycles of the DDR
controller. Rename it accordingly to give the reader a better idea
what this is about. While at it, at some documentation to struct
dmc_count_channel.

Link: https://lore.kernel.org/all/20231018061714.3553817-16-s.hauer@pengutronix.de/
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>