MAINTAINERS: rust: Add a new section for all of the time stuff

MAINTAINERS: rust: Add a new section for all of the time stuff

Add a new section for all of the time stuff to MAINTAINERS file, with
the existing hrtimer entry fold.

Acked-by: John Stultz <jstultz@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://lore.kernel.org/r/20250423192857.199712-7-fujita.tomonori@gmail.com
[ Changed T: entry branch to `timekeeping-next` - Andreas ]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>