mtd: physmap_of: Retire Gemini pad control

I wrote a proper pin control driver for the Gemini. Retire this
SoC-specific pad control and rely on the pin controller to manage
this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>