scripts/dtc: Re-sync with Linux 4.17-rc4

scripts/dtc: Re-sync with Linux 4.17-rc4

To achieve a DTC state (more or less) equivalent to Linux 4.17-rc4,
backport the DTC-specific part from below Linux commits:

b23d1a241f4eb4 ("kbuild: add %.lex.c and %.tab.[ch] to 'targets' automatically")
9a8dfb394c0467 ("kbuild: clean up *.lex.c and *.tab.[ch] patterns from top-level Makefile")
59889300274569 (".gitignore: move *.lex.c *.tab.[ch] patterns to the top-level .gitignore")
e039139be8c251 ("scripts/dtc: generate lexer and parser during build instead of shipping")
b24413180f5600 ("License cleanup: add SPDX GPL-2.0 license identifier to files with no license")

With this commit, the diff between Linux and U-boot common DTC files is:
$ git diff --stat --diff-filter=M v4.17-rc4 <this-commit> -- scripts/dtc
  scripts/dtc/.gitignore | 2 +-
  scripts/dtc/Makefile   | 3 +++

The delta is coming from U-boot commits:
* v2017.11 de163ecedb8a ("scripts: dtc: Add .gitignore")
* v2018.01 15b97f5c5e6d ("pylibfdt: move pylibfdt to scripts/dtc/pylibfdt and refactor makefile")

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>