product/morello: Updating CSS clock structure and frequency for CPUs in FVP

The patch 93e38fb4 aligned the CPUs'
frequency levels in FVP with those in SoC. This patch aligns the CSS
clock rate for CPUs in FVP referring to the structure used in SoC and
also updating the DVFS structure as that in SoC.

Signed-off-by: Himanshu Sharma <Himanshu.Sharma@arm.com>
Change-Id: Idc27bfb761c68b2cf02c5042f184e3acf438741f