gpio/xilinx: Create a single gpio chip on dual cores

Currently, we had two gpio chips on cores configured as dual.

This lead to mapping the same memory region twice and duplicating the
init and remove code.

This patch creates a single gpiochip for single and dual cores.

Suggested-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>