Various and sundry cleanups to the sample PCI driver:

• Split the first section into sub-sections instead of just paragraphs.
• Include a makefile for a kernel module for the sample driver and explain how to use 'make', 'make load', and 'make unload'.
• Use 'struct cdev *' rather than 'dev_t'.
• Add missing d_version to cdevsw.
• Explain how to use si_drv1 in struct cdev to get from a cdev back to the softc.
• Add a softc and move the cdev pointer into the softc. Given the cdev a unique name and minor number for each device.
• Use better comments in several places.
• Actually call device_set_desc() in probe, and use BUS_PROBE_DEFAULT rather than 0.
• Add missing call to destroy_dev() in detach.
• Use DEFINE_CLASS_0.

PR: docs/90830
Submitted by: Marius Nuennerich marius dot nuennerich at gmx dot net