gpio: sifive: Set affinity callback to parent

gpio: sifive: Set affinity callback to parent

This assigns the .irq_set_affinity to the parent callback.
I assume the sifive GPIO can be used in systems with
SMP.

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

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Yash Shah <yash.shah@sifive.com>
Cc: Wesley W. Terpstra <wesley@sifive.com>
Link: https://lore.kernel.org/r/20201117213351.249668-1-linus.walleij@linaro.org