Upstream: soc-acpi: add constant ops field

[plbossart]

Feedback from Takashi Sakamoto. Currently we store the ops in a void *pdata structure. we can add a dedicated ops field and keep the const qualifier.

[libinyang]

a RFC patch is submitted
#457

I'm not sure if my understanding is right or not. It seems we don't need a new member as we will not change it when running, do we?