Alter grammar to allow multiple /dts-v1/ tags

This patch allows dtc to accept multiple /dts-v1/ tags (provided they're
all at the beginning of the input), rather than giving a syntax error.

This makes it more convenient to include one .dts file from another without
having to be careful that the /dts-v1/ tag is in exactly one of them.

We a couple of existing testcases to take advantage of this, which
simplifies them slightly.

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