serial: 8250_pnp: Support configurable clock frequency

ACPI-based Loongson boards need configurable rather than fixed clock
frequency for serial ports.
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/r/20210715061637.134436-1-chenhuacai@loongson.cnSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_pnp.c

@@ -13,6 +13,7 @@
 #include <linux/pnp.h>
 #include <linux/string.h>
 #include <linux/kernel.h>
+#include <linux/property.h>
 #include <linux/serial_core.h>
 #include <linux/bitops.h>
 
@@ -475,6 +476,7 @@ serial_pnp_probe(struct pnp_dev *dev, const struct pnp_device_id *dev_id)
 	if (pnp_irq_flags(dev, 0) & IORESOURCE_IRQ_SHAREABLE)
 		uart.port.flags |= UPF_SHARE_IRQ;
 	uart.port.uartclk = 1843200;
+	device_property_read_u32(&dev->dev, "clock-frequency", &uart.port.uartclk);
 	uart.port.dev = &dev->dev;
 
 	line = serial8250_register_8250_port(&uart);