gitignore: Add more exlusions

litedram build directory used by the generator and the
verilator obj_dir can be taken out
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

.gitignore

@@ -8,3 +8,5 @@ tests/*/*.bin
 tests/*/*.hex
 tests/*/*.elf
 TAGS
+litedram/build/*
+obj_dir/*