MAINTAINERS: Update email address for David Gow

Update my email address for KUnit related things in MAINTAINERS (and add
an entry to .mailmap so nothing gets lost).

Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

.mailmap

@@ -214,6 +214,7 @@ Daniel Thompson <danielt@kernel.org> <daniel.thompson@linaro.org>
 Danilo Krummrich <dakr@kernel.org> <dakr@redhat.com>
 David Brownell <david-b@pacbell.net>
 David Collins <quic_collinsd@quicinc.com> <collinsd@codeaurora.org>
+David Gow <david@davidgow.net> <davidgow@google.com>
 David Heidelberg <david@ixit.cz> <d.okias@gmail.com>
 David Hildenbrand <david@kernel.org> <david@redhat.com>
 David Rheinsberg <david@readahead.eu> <dh.herrmann@gmail.com>