serial: ns16550: Support clocks via phandle

Previously ns16550 compatible UARTs probed via device tree have needed
their device tree nodes to contain a clock-frequency property. An
alternative to this commonly used with Linux is to reference a clock via
a phandle. This patch allows U-Boot to support that, retrieving the
clock frequency by probing the appropriate clock device.

For example, a system might choose to provide the UART base clock as a
reference to a clock common to multiple devices:

  sys_clk: clock {
    compatible = "fixed-clock";
    #clock-cells = <0>;
    clock-frequency = <10000000>;
  };

  uart0: uart@10000000 {
    compatible = "ns16550a";
    reg = <0x10000000 0x1000>;
    clocks = <&sys_clk>;
  };

  uart1: uart@10000000 {
    compatible = "ns16550a";
    reg = <0x10001000 0x1000>;
    clocks = <&sys_clk>;
  };

This removes the need for the frequency information to be duplicated in
multiple nodes and allows the device tree to be more descriptive of the
system.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/serial/ns16550.c

@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <clk.h>
 #include <dm.h>
 #include <errno.h>
 #include <fdtdec.h>
@@ -352,6 +353,8 @@ int ns16550_serial_ofdata_to_platdata(struct udevice *dev)
 {
 	struct ns16550_platdata *plat = dev->platdata;
 	fdt_addr_t addr;
+	struct clk clk;
+	int err;
 
 	/* try Processor Local Bus device first */
 	addr = dev_get_addr(dev);
@@ -397,9 +400,21 @@ int ns16550_serial_ofdata_to_platdata(struct udevice *dev)
 				     "reg-offset", 0);
 	plat->reg_shift = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
 					 "reg-shift", 0);
-	plat->clock = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
-				     "clock-frequency",
-				     CONFIG_SYS_NS16550_CLK);
+
+	err = clk_get_by_index(dev, 0, &clk);
+	if (!err) {
+		err = clk_get_rate(&clk);
+		if (!IS_ERR_VALUE(err))
+			plat->clock = err;
+	} else if (err != -ENODEV && err != -ENOSYS) {
+		debug("ns16550 failed to get clock\n");
+		return err;
+	}
+
+	if (!plat->clock)
+		plat->clock = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+					     "clock-frequency",
+					     CONFIG_SYS_NS16550_CLK);
 	if (!plat->clock) {
 		debug("ns16550 clock not defined\n");
 		return -EINVAL;