MAINTAINERS: add myself as reviewer for atomics

As I've fiddled about with the atomic infrastructure a fair bit now,
Peter suggested I should add myself as a reviewer or maintainer to make
sure I'm Cc'd on anything I might have an opinion on.

For now, add myself as a reviewer.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220104095018.1990058-1-mark.rutland@arm.com

MAINTAINERS

@@ -3193,6 +3193,7 @@ ATOMIC INFRASTRUCTURE
 M:	Will Deacon <will@kernel.org>
 M:	Peter Zijlstra <peterz@infradead.org>
 R:	Boqun Feng <boqun.feng@gmail.com>
+R:	Mark Rutland <mark.rutland@arm.com>
 L:	linux-kernel@vger.kernel.org
 S:	Maintained
 F:	arch/*/include/asm/atomic*.h