serial: 8250_of: Add reset support

This adds the hooks for an optional reset controller in the 8250 device
tree node.

Signed-off-by: Joel Stanley <[e-mail]>
Reviewed-by: Philipp Zabel <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/8250.txt

···
   property.
 - tx-threshold: Specify the TX FIFO low water indication for parts with
   programmable TX FIFO thresholds.
+- resets : phandle + reset specifier pairs
 
 Note:
 * fsl,ns16550:

drivers/tty/serial/8250/8250_of.c

···
 #include <linux/of_irq.h>
 #include <linux/of_platform.h>
 #include <linux/clk.h>
+#include <linux/reset.h>
 
 #include "8250.h"
 
 struct of_serial_info {
 	struct clk *clk;
+	struct reset_control *rst;
 	int type;
 	int line;
 };
···
 		}
 	}
 
+	info->rst = devm_reset_control_get_optional_shared(&ofdev->dev, NULL);
+	if (IS_ERR(info->rst))
+		goto out;
+	ret = reset_control_deassert(info->rst);
+	if (ret)
+		goto out;
+
 	port->type = type;
 	port->uartclk = clk;
 	port->flags = UPF_SHARE_IRQ | UPF_BOOT_AUTOCONF | UPF_IOREMAP
···
 
 	serial8250_unregister_port(info->line);
 
+	reset_control_assert(info->rst);
 	if (info->clk)
 		clk_disable_unprepare(info->clk);
 	kfree(info);