gpio: loongson: Create a dynamic platform device

It is pretty helpful to create some kind of device for backing the
GPIO chips, especially when preparing the driver for using
GENERIC_GPIO, so let's create a simple platform device and a simple
platform device driver and create the gpiochip in the .probe() routine
for the device driver. Keep all at the core initcall so the behaviour
is the same as before.

Cc: Keguang Zhang <keguang.zhang@gmail.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: linux-mips@linux-mips.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>