hwmon: (pcf8591) Documentation clean-ups

Clean up the pcf8591 driver documentation:
* The PCF8591 chip is now an NXP product.
* Fix a sysfs path.
* Fix the name of sysfs attributes.
* And a few other random fixes.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>