dt: Linux DT usage model documentation

v2: 2nd draft
 - Editorial cleanups (Randy Dunlap and Stephen Warren)
 - Added missing Microblaze reference (Stephen Neuendorffer)
 - Make example of platform_device creation clearer (Shawn Guo)
 - Expand on PowerPC history and mention i2c mess (David Gibson)
 - convert to plain text (remove bits of html formating)
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>