soc/tegra: pmc: Avoid crash for non-wake IRQs

For interrupts that are not wakeup sources but that may end up getting
mapped through the PMC as interrupt parent (this can happen for GPIOs),
return early in order to avoid a subsequent crash from an out-of-bounds
access to the register region.

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