MAINTAINERS: Add section for Renesas clock drivers

Add a section for Renesas clock drivers, as found on Renesas ARM SoCs,
and list myself as the maintainer.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>