of/vendor-prefixes: add Imagination Technologies

The "powervr" prefix which is currently described as "Imagination
Technologies" isn't really appropriate for non-PowerVR hardware, so
deprecate it, changing the description of "powervr" to "PowerVR
(deprecated, use img)", and add a separate "img" prefix for "Imagination
Technologies Ltd.".

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>