CSS: Fix overrun if system power level is not available

This patch fixes an array overrun in CSS scmi driver if the
system power domain level is less than 2. This was reported from
https://scan.coverity.com/projects/arm-software-arm-trusted-firmware



CID 308492

Change-Id: I3a59c700490816718d20c71141281f19b2b7e7f7
Signed-off-by: Soby Mathew <soby.mathew@arm.com>