hwmon: (sht15) move header to linux/platform_data/

This patch moves the sht15.h header from include/linux to
include/linux/platform_data, and update existing support (stargate2
platform) accordingly.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>