MAINTAINERS: Update Allwinner sunXi maintainer files

Add entries for all files having either sunxi or one of the Allwinner flavours
in their name.

Also elect Emilio Lopez as the maintainer of the clock driver, since he was
already acting like it.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>