sensor/lis2mdl: use kconfig named choice for TRIGGER_MODE

Use Kconfig named choice for TRIGGER_MODE in order to easy
up the trigger mode selection in default configurations
for boards using this sensor driver.

Signed-off-by: Armando Visconti <armando.visconti@st.com>
diff --git a/drivers/sensor/lis2mdl/Kconfig b/drivers/sensor/lis2mdl/Kconfig
index 1e0126c..7072334 100644
--- a/drivers/sensor/lis2mdl/Kconfig
+++ b/drivers/sensor/lis2mdl/Kconfig
@@ -12,7 +12,7 @@
 
 if LIS2MDL
 
-choice
+choice LIS2MDL_TRIGGER_MODE
 	prompt "Trigger mode"
 	default LIS2MDL_TRIGGER_GLOBAL_THREAD
 	help
@@ -31,7 +31,7 @@
 	depends on GPIO
 	select LIS2MDL_TRIGGER
 
-endchoice
+endchoice # LIS2MDL_TRIGGER_MODE
 
 config LIS2MDL_TRIGGER
 	bool