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