MAINTAINERS: Update the Gemini maintainer list

This patch:
- Adds myself as comaintainer for the Gemini.
- Adds the Gemini main bindings to the file list.
- Adds the pin controller plus bindings to the Gemini
  file list.
- Fixes up the path of the RTC binding and driver.

Acked-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>