css_pm_scmi: optimise cpu suspend to remove redundant scmi call

In css platforms where the cpu/cluster management is done by the hardware,
software does need to issue certain scmi requests.

This patch wraps those scmi calls around the HW_ASSISTED_COHERENCY
build option to remove them on platforms that have this hardware support.

Change-Id: Ie818e234484ef18549aa7f977aef5c3f0fc26c27
Signed-off-by: Deepak Pandey <Deepak.Pandey@arm.com>
Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Signed-off-by: Nariman Poushin <nariman.poushin@linaro.org>