Add 2x2 block unit syntax

Add 2x2 block size syntax to the codec system. It prepares for the
4x4 coding block unit for 420 format. This change retains the same
coding statistics.

Change-Id: If8e9a31bd6b4b75bc994539dc4dd8021d455ba57
diff --git a/av1/encoder/rd.c b/av1/encoder/rd.c
index 190e07e..5477706 100644
--- a/av1/encoder/rd.c
+++ b/av1/encoder/rd.c
@@ -63,6 +63,9 @@
 // This table is used to correct for block size.
 // The factors here are << 2 (2 = x0.5, 32 = x8 etc).
 static const uint8_t rd_thresh_block_size_factor[BLOCK_SIZES] = {
+#if CONFIG_CB4X4
+  2,  2,  2,
+#endif
   2,  3,  3, 4, 6, 6, 8, 12, 12, 16, 24, 24, 32,
 #if CONFIG_EXT_PARTITION
   48, 48, 64