thermal: Constify 'type' argument for the registration routine

thermal_zone_device_register() does not modify 'type' argument, so it is
safe to declare it as const. Otherwise, if we pass a const string, we are
getting the ugly warning:

CC drivers/power/power_supply_core.o
drivers/power/power_supply_core.c: In function 'psy_register_thermal':
drivers/power/power_supply_core.c:204:6: warning: passing argument 1 of 'thermal_zone_device_register' discards 'const' qualifier from pointer target type [enabled by default]
include/linux/thermal.h:140:29: note: expected 'char *' but argument is of type 'const char *'

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: Jean Delvare <khali@linux-fr.org>