UIO: Add missing documentation of features added recently

The following features were added to the UIO framework in the near past:

* Generic drivers for platform devices (uio_pdrv, uio_pdrv_genirq)
* an "offset" sysfs attribute for memory mappings

Unfortunately, all this went in without documentation (won't happen again...)

This patch updates UIO documentation.

Signed-off-by: Hans J. Koch <hjk@linutronix.de>
Acked-by: Uwe Kleine-König <ukleinek@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>