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.ukSigned-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>

.mailmap

@@ -70,6 +70,8 @@ Baolin Wang <baolin.wang@linux.alibaba.com> <baolin.wang@unisoc.com>
 Baolin Wang <baolin.wang@linux.alibaba.com> <baolin.wang7@gmail.com>
 Bart Van Assche <bvanassche@acm.org> <bart.vanassche@sandisk.com>
 Bart Van Assche <bvanassche@acm.org> <bart.vanassche@wdc.com>
+Ben Dooks <ben-linux@fluff.org> <ben.dooks@simtec.co.uk>
+Ben Dooks <ben-linux@fluff.org> <ben.dooks@sifive.com>
 Ben Gardner <bgardner@wabtec.com>
 Ben M Cahill <ben.m.cahill@intel.com>
 Ben Widawsky <bwidawsk@kernel.org> <ben@bwidawsk.net>