Tegra: implement common handler `plat_get_target_pwr_state()`

Tegra: implement common handler `plat_get_target_pwr_state()`

This patch adds a platform handler to calculate the proper target power
level at the specified affinity level.

Tegra platforms assign a local state value in order of decreasing depth
of the power state i.e. for two power states X & Y, if X < Y then X
represents a shallower power state than Y. As a result, the coordinated
target local power state for a power domain will be the maximum of the
requested local power state values.

Change-Id: I67360684b7f5b783fcfdd605b96da5375fa05417
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>