Add 2x2 variance function

Change-Id: I73bcb8ab5727e2d07e34ca35e9e014f3c6f63d56
diff --git a/aom_dsp/variance.c b/aom_dsp/variance.c
index c93f98e..b9b6741 100644
--- a/aom_dsp/variance.c
+++ b/aom_dsp/variance.c
@@ -233,6 +233,7 @@
 VARIANCES(8, 4)
 VARIANCES(4, 8)
 VARIANCES(4, 4)
+VARIANCES(2, 2)
 
 GET_VAR(16, 16)
 GET_VAR(8, 8)