Extend nRF51 AnalogIn voltage range to 3.6 V

Previous the voltage range was set to 1.2 from SoC internal reference source.
This caused problem with testing. It is unexpected that range is much
shorter than vdd as well.

The voltage range was extended using SoC build in analog prescaler (set to 1/3).

Author: nvlsianpu

targets/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF51822_UNIFIED/analogin_api.c

@@ -67,7 +67,7 @@ uint16_t analogin_read_u16(analogin_t *obj)
 
     // initialization by assigment because IAR dosen't support variable initializer in declaration statement.
     adc_channel.config.config.resolution = NRF_ADC_CONFIG_RES_10BIT;
-    adc_channel.config.config.input      = NRF_ADC_CONFIG_SCALING_INPUT_FULL_SCALE;
+    adc_channel.config.config.input      = NRF_ADC_CONFIG_SCALING_INPUT_ONE_THIRD;
     adc_channel.config.config.reference  = NRF_ADC_CONFIG_REF_VBG;
     adc_channel.config.config.ain        = (obj->adc_pin);
     adc_channel.p_next = NULL;