Implement get_sys_suspend_power_state() handler for Tegra

Implement get_sys_suspend_power_state() handler for Tegra

This patch implements the get_sys_suspend_power_state() handler required by
the PSCI SYSTEM_SUSPEND API. The intent of this handler is to return the
appropriate State-ID field which can be utilized in `affinst_suspend()` to
suspend to system affinity level.

Signed-off-by: Varun Wadekar <[email protected]>