Add config key for amplitude.

This is intended for setting (or getting) the amplitude of a source
which doesn't really have an MQ associated with it, such as the demo
driver's analog channels.

include/libsigrok/libsigrok.h

@@ -756,6 +756,9 @@ enum sr_configkey {
 	/** Choice of clock edge for external clock ("r" or "f"). */
 	SR_CONF_CLOCK_EDGE,
 
+	/** Amplitude of a source without strictly-defined MQ. */
+	SR_CONF_AMPLITUDE,
+
 	/*--- Special stuff -------------------------------------------------*/
 
 	/** Scan options supported by the driver. */

src/hwdriver.c

@@ -124,6 +124,8 @@ static struct sr_config_info sr_config_info_data[] = {
 		"Sample limit", NULL},
 	{SR_CONF_CLOCK_EDGE, SR_T_STRING, "clock_edge",
 		"Clock edge", NULL},
+	{SR_CONF_AMPLITUDE, SR_T_FLOAT, "amplitude",
+		"Amplitude", NULL},
 	{0, 0, NULL, NULL, NULL},
 };