mod_scmi_power_domain: Add header file

A module configuration definition is added in preparation
for adding debug support.

Change-Id: Icc0027faa88bc735579068ebd8fd617b1de752ea
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>