scmi_power_domain: Split configuration from implementation

With this patch the configuration for this module becomes a
separate file for each platform, like all the other modules.

Change-Id: If001c668d731c28f225ef188ebe097120f41d74a
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>