ARM: tegra: pm: add platform suspend support

Adding suspend to RAM support for Tegra platform. There are three suspend
mode for Tegra. The difference were below.

* LP2: CPU voltage off
* LP1: CPU voltage off, DRAM in self-refresh
* LP0: CPU + Core voltage off, DRAM in self-refresh

After this patch, the LP2 suspend mode will be supported.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>