hwmon: (lm87) Add support for configuration through platform_data

The lm87 driver normally assumes that firmware configured the chip
correctly.  Since this is not always the case, alllow platform code to
set the channel register value via platform_data.  All other
configuration registers can be changed after driver initialisation.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>