dtc: Clean up /dts-v1/ and /plugin/ handling in grammar

First remove the non-terminal name 'versioninfo' - /plugin/ doesn't really
indicate a "version" per se, and version could be confused with the dtb
output version.

Second allow the /dts-v1/; /plugin/; sequence to be repeated, for easier
use of include files - but ensure that all copies match, so you can't
include a file declaring /plugin/ in one that doesn't, or vice versa.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>