PM / devfreq: rockchip-dfi: Embed desc into private data struct

PM / devfreq: rockchip-dfi: Embed desc into private data struct

No need for an extra allocation, just embed the struct
devfreq_event_desc into the private data struct.

Link: https://lore.kernel.org/lkml/20230704093242.583575-3-s.hauer@pengutronix.de/
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
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>