hw/gpio: Add in AST2600 specific implementation

The AST2600 has the same sets of 3.6v gpios as the AST2400 plus an
addtional two sets of 1.8V gpios.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Joel Stanley <joel@jms.id.au>
Message-id: 20190925143248.10000-15-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>