drm/imagination: Add skeleton PowerVR driver

This adds the basic skeleton of the driver. The driver registers
itself with DRM on probe. Ioctl handlers are currently implemented
as stubs.

Changes since v8:
- Corrected license identifiers

Changes since v5:
- Update compatible string & description to match marketing name
- Checkpatch fixes in to/from_pvr_device/file macros

Changes since v3:
- Clarify supported GPU generations in driver description
- Use drm_dev_unplug() when removing device
- Change from_* and to_* functions to macros
- Fix IS_PTR/PTR_ERR confusion in pvr_probe()
- Remove err_out labels in favour of direct returning
- Remove specific am62 compatible match string
- Drop MODULE_FIRMWARE()

Co-developed-by: Frank Binns <frank.binns@imgtec.com>
Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Co-developed-by: Matt Coster <matt.coster@imgtec.com>
Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Signed-off-by: Sarah Walker <sarah.walker@imgtec.com>
Signed-off-by: Donald Robson <donald.robson@imgtec.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>