MAINTAINERS: Update Paul Walmsley's E-mail address

My experiment with using corporate Gmail for Linux kernel list
interaction has come to an end.  For my MAINTAINERS entries that
use that E-mail address, let's switch those to use the k.org E-mail
forwarding.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <pjw@kernel.org>