libexternal: Add 3840x2160p25Hz to getAttrForMode

Fix the black screen shown on TV when Resolution
3840x2160p@25Hz was set

Change-Id: Ib3ff35ce53df0409c7643f349f8095c7f2ad4d1d
diff --git a/libexternal/external.cpp b/libexternal/external.cpp
index 43451de..e3de5ce 100644
--- a/libexternal/external.cpp
+++ b/libexternal/external.cpp
@@ -781,6 +781,11 @@
             height = 2160;
             fps = 24;
             break;
+        case HDMI_VFRMT_3840x2160p25_16_9:
+            width = 3840;
+            height = 2160;
+            fps = 25;
+            break;
         case HDMI_VFRMT_3840x2160p30_16_9:
             width = 3840;
             height = 2160;