gpio: mt7621: Kill parent_device usage

This gpio controller sets the parent_device field, but doesn't have
any runtime PM functionality. Get rid of it.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>
Link: https://lore.kernel.org/r/20220201120310.878267-7-maz@kernel.org