treewide: fix transposed "sign" typos and update spelling.txt

treewide: fix transposed "sign" typos and update spelling.txt

Several comments transpose the letters in "assigned" and "unsigned",
spelling them with "sing" instead of "sign".  Correct all of them.

Of these, the misspelling of "assigned" is not yet flagged by checkpatch,
so also add it to scripts/spelling.txt.

The remaining matches of `grep -ri singed` are RISINGEDGE register and
enum names, not typos.

Link: https://lore.kernel.org/20260612181633.734458-1-iamsharduld@gmail.com
Signed-off-by: Shardul Deshpande <iamsharduld@gmail.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>