This patch reduces the code complexity for the platform's 'get_target_pwr_state'
handler, by reducing the number of 'if' conditions and adding helper functions
to calculate power state for the cluster/system.

Tested with 'pmccabe'

Change-Id: I32fa4c814bd97f620f2003fa39f1bfceae563771
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>