Add PEC support to the lm90 driver. Only the ADM1032 chip supports it,
and in a rather tricky way, which is why this patch comes with
documentation reinforcements. At least, this demonstrates that the new
PEC support logic in i2c-core can properly deal with chips with partial
PEC support.

As enabling PEC causes a significant performance drop, it can be
disabled through a sysfs file (unsurprisingly named "pec").
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>