TLV: add TLVP_PRES_LEN() definition to check for existance + minimum length

This is an import from osmo-bts where it was introduced in
6b561bb7ba5f10c5fd96542a7e42de2822352a67
diff --git a/include/osmocom/gsm/tlv.h b/include/osmocom/gsm/tlv.h
index fda1810..d54ef1a 100644
--- a/include/osmocom/gsm/tlv.h
+++ b/include/osmocom/gsm/tlv.h
@@ -409,6 +409,9 @@
 #define TLVP_LEN(x, y)		(x)->lv[y].len
 #define TLVP_VAL(x, y)		(x)->lv[y].val
 
+#define TLVP_PRES_LEN(tp, tag, min_len) \
+	(TLVP_PRESENT(tp, tag) && TLVP_LEN(tp, tag) >= min_len)
+
 /*! \brief Align given TLV element with 16 bit value to an even address
  *  \param[in] tp pointer to \ref tlv_parsed
  *  \param[in] pos element to return