ab8500_btemp: Make ab8500_btemp_get* interfaces public

Make ab8500_btemp_get_temp interface public, export it and also export the
ab8500_btemp_get, ab8500_btemp_get_batctrl_temp interfaces, so that the
ab8500 hwmon driver can use them.

Signed-off-by: Hongbo Zhang <hongbo.zhang@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>