soc/tegra: pmc: Fail to allocate more than one wake IRQ

The code currently doesn't support allocating more than one wake IRQ at
a time. Detect this situation and error out. Also make sure to output a
warning when that happens to help track down callers.

Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>