gpio: zynq: Setup chip->base based on alias ID

In past Xilinx gpio-zynq driver was setting up gpio chip->base as 0
which was chagned to autodetection when driver was upstreamed. Older
systems, which were using this old version, setup SW stack which expects
zynq gpio base as 0 and right now there is no way how to set this up.

The patch is adding an option to setup chip->base based on aliases which
is something what some other drivers are doing too.
It means when gpio0 alias is setup then chip->base is 0. When gpio alias
is not setup gpiochip_find_base() set it up properly which is current
behavior.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-zynq.c

@@ -827,7 +827,7 @@ static int zynq_gpio_probe(struct platform_device *pdev)
 	chip->free = zynq_gpio_free;
 	chip->direction_input = zynq_gpio_dir_in;
 	chip->direction_output = zynq_gpio_dir_out;
-	chip->base = -1;
+	chip->base = of_alias_get_id(pdev->dev.of_node, "gpio");
 	chip->ngpio = gpio->p_data->ngpio;
 
 	/* Retrieve GPIO clock */