gpio: aspeed: Update documentation with ast2600 controllers

gpio: aspeed: Update documentation with ast2600 controllers

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 36 1.8V GPIO pins.  We use
the ngpio property to differentiate between these controllers.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Link: https://lore.kernel.org/r/20190906062547.13264-1-rashmica.g@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>