ARM: tegra: add pending SGI checking API

The "powered-down" CPU idle mode of Tegra cut off the vdd_cpu rail, it
include the power of GIC. That caused the SGI (Software Generated
Interrupt) been lost. Because the SGI can't wake up the CPU that in
the "powered-down" CPU idle mode. We need to check if there is any
pending SGI when go into "powered-down" CPU idle mode. This is important
especially when applying the coupled cpuidle framework into "power-down"
cpuidle dirver. Because the coupled cpuidle framework may have the
chance that misses IPI_SINGLE_FUNC handling sometimes.

For the PPI or SPI, something like the legacy peripheral interrupt. It
still can be maintained by Tegra legacy interrupt controller. If there
is any pending PPI or SPI when CPU in "powered-down" CPU idle mode. The
CPU can be woken up immediately. So we don't need to take care the same
situation for PPI or SPI.

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