MAINTAINERS: Update my email address to @kernel.org

I will soon lose access to my @arm.com email address, so let's
update the MAINTAINERS file to point to my @kernel.org address,
as well as .mailmap for good measure.

Note that my @arm.com address will still work, but someone else
will be reading whatever is sent there. Don't say you didn't know!

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>