pinctrl: introduce generic pin config

pinctrl: introduce generic pin config

This is a split-off from the earlier patch set which adds generic
pin configuration for the pin controllers that want it. Since
we may have a system with mixed generic and custom pin controllers,
we pass a boolean in the pin controller ops vtable to indicate
if it is generic.

ChangeLog v1->v5:
- Follow parent patch versioning number system.
- Document the semantic meaning of return values from pin config
  get functions, so we can iterate over pins and check their
  properties from debugfs as part of the generic config code.
- Use proper cast functions in the generic debugfs pin config
  file.
- Expand generic config to optionally cover groups too.
ChangeLog v5->v6:
- Update to match underlying changes.
ChangeLog v6->v7:
- Drop DRIVE_OFF parameter, use bias high impedance for this
- Delete argument for drive modes push-pull, od and os. These
  are now just state transitions.
- Delete slew rate rising/falling due to discussions on on
  proper semantics
- Drop config wakeup, struct irq_chip does this for now, add
  back if need be.
- Set PIN_CONFIG_END to 0x7fff making room for custom config
  parameters from 0x8000 and up.
- Prefix accessor functions with pinconf_