gpio: aspeed: Add in ast2600 details to Aspeed driver

The ast2600 is a new generation of SoC from ASPEED. Similarly to the
ast2400 and ast2500, it has a GPIO controller for it's 3.3V GPIO pins.
Additionally, it has a GPIO controller for 1.8V GPIO pins.

As the register names for both controllers are the same and the 36 1.8V
GPIOs and the first 36 of the 3.3V GPIOs are all bidirectional, we can
use the same configuration struct and use the ngpio property to
differentiate between the two sets of GPIOs.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Link: https://lore.kernel.org/r/20190906063737.15428-1-rashmica.g@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>