serial: 8250_dw: Add optional reset control support

The Allwinner A31 and A23 SoCs have a reset controller
maintaining the UART in reset by default.

This patch adds optional reset support to the driver.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/snps-dw-apb-uart.txt

@@ -15,6 +15,7 @@ The supplying peripheral clock can also be handled, needing a second property
 	Required elements: "baudclk", "apb_pclk"
 
 Optional properties:
+- resets : phandle to the parent reset controller.
 - reg-shift : quantity to shift the register offsets by.  If this property is
   not present then the register offsets are not shifted.
 - reg-io-width : the size (in bytes) of the IO accesses that should be

drivers/tty/serial/8250/8250_dw.c

@@ -26,6 +26,7 @@
 #include <linux/slab.h>
 #include <linux/acpi.h>
 #include <linux/clk.h>
+#include <linux/reset.h>
 #include <linux/pm_runtime.h>
 
 #include <asm/byteorder.h>
@@ -60,6 +61,7 @@ struct dw8250_data {
 	int			line;
 	struct clk		*clk;
 	struct clk		*pclk;
+	struct reset_control	*rst;
 	struct uart_8250_dma	dma;
 };
 
@@ -383,6 +385,10 @@ static int dw8250_probe(struct platform_device *pdev)
 		}
 	}
 
+	data->rst = devm_reset_control_get_optional(&pdev->dev, NULL);
+	if (!IS_ERR(data->rst))
+		reset_control_deassert(data->rst);
+
 	data->dma.rx_chan_id = -1;
 	data->dma.tx_chan_id = -1;
 	data->dma.rx_param = data;
@@ -426,6 +432,9 @@ static int dw8250_remove(struct platform_device *pdev)
 
 	serial8250_unregister_port(data->line);
 
+	if (!IS_ERR(data->rst))
+		reset_control_assert(data->rst);
+
 	if (!IS_ERR(data->pclk))
 		clk_disable_unprepare(data->pclk);