hwdriver: Add configuration key for number of powerline cycles

High precision multimeters have a special setting, called "number of
powerline cycles" (NPLC) which determines the integration time of the
ADC in terms of the power line period. Some devices need their NPLC
adjusted from the default value before they can measure at their full
rated precision.

include/libsigrok/libsigrok.h

@@ -1015,6 +1015,9 @@ enum sr_configkey {
 	/** The device supports setting a probe factor. */
 	SR_CONF_PROBE_FACTOR,
 
+	/** Number of powerline cycles for ADC integration time. */
+	SR_CONF_ADC_POWERLINE_CYCLES,
+
 	/* Update sr_key_info_config[] (hwdriver.c) upon changes! */
 
 	/*--- Acquisition modes, sample limiting ----------------------------*/

src/hwdriver.c

@@ -189,6 +189,8 @@ static struct sr_key_info sr_key_info_config[] = {
 		"Data source", NULL},
 	{SR_CONF_PROBE_FACTOR, SR_T_UINT64, "probe_factor",
 		"Probe factor", NULL},
+	{SR_CONF_ADC_POWERLINE_CYCLES, SR_T_FLOAT, "nplc",
+		"Number of ADC powerline cycles", NULL},
 
 	/* Acquisition modes, sample limiting */
 	{SR_CONF_LIMIT_MSEC, SR_T_UINT64, "limit_time",