script: dtc: clean generated files

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>