Tegra: Add support for fake system suspend

This patch adds support for fake system suspend (SC7).
This is a debug mode, to ensure that a different code path is
executed for cases like pre-silicon development, where a
full-fledged SC7 is not possible in early stages.

This particular patch ensures that, if fake system suspend is
enabled (denoted by tegra_fake_system_suspend variable
having a non-zero value), instead of calling WFI, a request
for a warm reset is made for starting the SC7 exit procedure.

This ensures that the code path of kernel->ATF and back to
kernel is executed without depending on other components
involved in SC7 code path.

Additionally, this patch also adds support for SMC call
from kernel, enabling fake system suspend mode.

Signed-off-by: Vignesh Radhakrishnan <vigneshr@nvidia.com>
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>