gpio: aspeed: Make bank names strings

The Aspeed SoCs have more GPIOs than can be represented with A-Z. The
documentation uses two letter names such as AA and AB, so make the names
a three-character array in the bank struct to accommodate this.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>