This is the same as abx500_adc_therm, but when the former is used,
the following warning flood pops up:

drivers/power/ab8500_btemp.c: In function 'ab8500_btemp_batctrl_volt_to_res':
ab8500_btemp.c:150:25: warning: comparison between 'enum abx500_adc_therm' and 'enum adc_therm' [-Wenum-compare]
ab8500_btemp.c: In function 'ab8500_btemp_curr_source_enable':
ab8500_btemp.c:212:25: warning: comparison between 'enum abx500_adc_therm' and 'enum adc_therm' [-Wenum-compare]
ab8500_btemp.c:244:32: warning: comparison between 'enum abx500_adc_therm' and 'enum adc_therm' [-Wenum-compare]
ab8500_btemp.c: In function 'ab8500_btemp_measure_temp':
ab8500_btemp.c:462:25: warning: comparison between 'enum abx500_adc_therm' and 'enum adc_therm' [-Wenum-compare]
ab8500_btemp.c: In function 'ab8500_btemp_id':
ab8500_btemp.c:528:121: warning: comparison between 'enum abx500_adc_therm' and 'enum adc_therm' [-Wenum-compare]
ab8500_btemp.c:551:25: warning: comparison between 'enum abx500_adc_therm' and 'enum adc_therm' [-Wenum-compare]

This patch fixes the issue by switching the driver to use more
namespace-friendly enum.
Signed-off-by: NAnton Vorontsov <anton.vorontsov@linaro.org>