Tegra: per-soc `get_target_pwr_state` handler

Tegra: per-soc `get_target_pwr_state` handler

This patch implements a per-soc handler to calculate the target
power state for the cluster/system. A weak implementation of the
handler is provided for platforms to use by default.

For SoCs with multiple CPU clusters, this handler would provide
the individual cluster/system state, allowing the PSCI service to
flush caches during cluster/system power down.

Change-Id: I568cdb42204f9841a8430bd9105bd694f71cf91d
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>