MAINTAINERS: .mailmap: update Sumit Garg's email address

Update Sumit Garg's email address to @kernel.org.

Link: https://lkml.kernel.org/r/20250227113228.1809449-1-sumit.garg@linaro.org


Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>