Add value strings for Probable Cause Type

Add string representation of Probable Cause Type from 3GPP TS 12.21 ยง
9.4.43.

Change-Id: I9fe14ed3b5398f59dd06a509e4d419e074cc20a7
Related: OS#1615
diff --git a/src/gsm/abis_nm.c b/src/gsm/abis_nm.c
index 2de4941..f50a54f 100644
--- a/src/gsm/abis_nm.c
+++ b/src/gsm/abis_nm.c
@@ -164,6 +164,13 @@
 	{ 0, NULL }
 };
 
+const struct value_string abis_nm_pcause_type_names[] = {
+	{ NM_PCAUSE_T_X721,	"ISO/CCITT values (X.721)"},
+	{ NM_PCAUSE_T_GSM,	"GSM specific values"},
+	{ NM_PCAUSE_T_MANUF,	"Manufacturer specific values"},
+	{ 0, NULL }
+};
+
 /* Chapter 9.4.36 */
 static const struct value_string nack_cause_names[] = {
 	/* General Nack Causes */