scripts: dtc: Add .gitignore

Ignore these generated files during the build of dtc.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

scripts/dtc/.gitignore

@@ -0,0 +1,4 @@
+/dtc
+/dtc-lexer.lex.c
+/dtc-parser.tab.c
+/dtc-parser.tab.h