mailmap: add entries for Ben Dooks

I am going to be losing my sifive.com address soon and I also realised my
old Simtec address (from >10 years ago) is also not been updates so update
.mailmap for both.

Link: https://lkml.kernel.org/r/20230615081820.79485-1-ben.dooks@codethink.co.uk


Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>