mailmap: add entry for John Keeping

Map my corporate address to my personal one, as I am leaving the
company.

Link: https://lkml.kernel.org/r/20230531144839.1157112-1-john@keeping.me.uk


Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>