MAINTAINERS: Add DT Bindings for Renesas Clock Generators

The "RENESAS CLOCK DRIVERS" section lacks the related DT bindings.
Add them.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20200417141116.23019-1-geert+renesas@glider.be