scmi_system_power: Move the graceful request into the policy function.
Created by: tarek-arm
This change allows each platform to define a policy for handling graceful requests.
Change-Id: I3dc5ed0cf57c859a2a73a2a9a0f8ffc3bd311031 Signed-off-by: Tarek El-Sherbiny tarek.el-sherbiny@arm.com