scripts/dtc: generate lexer and parser during build instead of shipping

Now that the kernel build supports flex and bison, remove the _shipped
files and generate them during the build instead.

Based on Masahiro's original patch.

Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>