MAINTAINERS: Update my email address to use @kernel.org

My @arm.com address will stop working at the end of August, so update to
my @kernel.org address where you'll still be able to reach me.

When I say "stop working" I really mean "will go to my line manager", so
send patches there at your peril because they may reply with roadmaps
and spreadsheets. You have been warned.

Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: arm-soc <arm@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>