cut branch for tikv-5.2

Signed-off-by: Yi Wu <yiwu@pingcap.com>

.gitmodules

 [submodule "rocksdb"]
 	path = librocksdb_sys/rocksdb
 	url = https://github.com/tikv/rocksdb.git
-	branch = 6.4.tikv
+	branch = tikv-5.2
 
 [submodule "titan"]
 	path = librocksdb_sys/libtitan_sys/titan
 	url = https://github.com/tikv/titan.git
-	branch = master
+	branch = tikv-5.2

titan @ 2d178df0

-Subproject commit ecb5cba016096309cbd9b566bdc4ce62307527a0
+Subproject commit 2d178df0f07f09300c3018663591eef1a44a3303