Clean up the naming of pinmux-related objects:
* Refer to drive groups rather than pad groups to match the Linux kernel.
* Ensure all pinmux API types are prefixed with pmux_, values (defines)
  are prefixed with PMUX_, and functions prefixed with pinmux_.
* Modify a few type names to make their content clearer.
* Minimal changes to SoC-specific .h/.c files are made so the code still
  compiles. A separate per-SoC change will be made immediately following,
  in order to keep individual patch size down.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>