gpio: aspeed: Use ngpio property from device tree if available

Use the ngpio property from the device tree if it exists. If it doesn't
then fallback to the hardcoded value in the config.

This is in preparation for adding ast2600 support. The ast2600 SoC has
two GPIO controllers and so requires two instances of the GPIO driver.
We use the ngpio property to different between them as they have
different numbers of GPIOs.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Link: https://lore.kernel.org/r/20190906062727.13521-1-rashmica.g@gmail.com
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>