MAINTAINERS: Add DT Bindings for Renesas Pin Function Controllers

The "PIN CONTROLLER - RENESAS" section lacks the related DT bindings.
Add them.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200417141003.22816-1-geert+renesas@glider.be