MAINTAINERS: add maintainer for C-SKY drivers

There are two intc drivers and two clocksource drivers, also include
related dt-bindings' documentations.

Change ren_guo@c-sky.com to guoren@kernel.org

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>