PM / devfreq: rockchip-dfi: Make pmu regmap mandatory

As a matter of fact the regmap_pmu already is mandatory because
it is used unconditionally in the driver. Bail out gracefully in
probe() rather than crashing later.

Link: https://lore.kernel.org/lkml/20230704093242.583575-2-s.hauer@pengutronix.de/


Fixes: b9d1262b ("PM / devfreq: event: support rockchip dfi controller")
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>