tests: dtoc: adapt tests to changed dtoc output

The dtoc tests need to be adapted to dtoc being changed to output platdata
structs as const, which has been introduced in commit 7d05d3a8 ("dtoc:
make generated platdata structs const").

Fixes: 7d05d3a8 ("dtoc: make generated platdata structs const")
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>