gpio: tegra186: Set affinity callback to parent

This assigns the .irq_set_affinity to the parent callback.
I assume the Tegra186 is an SMP system so this would be
beneficial.

I used the pattern making the hirerarchy tolerant for missing
parent as in Marc's earlier patch.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Vidya Sagar <vidyas@nvidia.com>
Link: https://lore.kernel.org/r/20201117213351.249668-2-linus.walleij@linaro.org