soc: rockchip: power-domain: check the existing of regmap

Check return value of syscon_node_to_regmap for
rockchip_pm_domain_probe. If err value is returned, probe
procedure should abort.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>