gpio: Add Aspeed driver

The Aspeed SoCs contain GPIOs banked by letter, where each bank contains
8 pins. The GPIO banks are then grouped in sets of four in the register
layout.

The implementation exposes multiple banks through the one driver and
requests and releases pins via the pinctrl subsystem. The hardware
supports generation of interrupts from all GPIO-capable pins.

A number of hardware features are not yet supported: Configuration of
interrupt direction (ARM or LPC), debouncing, and WDT reset tolerance
for output ports.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>