Fix "make distclean" to clean up generated dtc files.
Without this patch the following files are left around:
- dtc-lexer.lex.c
- dtc-parser.tab.c
- dtc-parser.tab.h
Signed-off-by: Magnus Damm <damm@opensource.se>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>