serial: liteuart: Add support for earlycon

Most litex boards using RISC-V soft cores us the sbi earlycon, however
this is not available for non RISC-V litex SoC's. This patch enables
earlycon for liteuart which is available on all Litex SoC's making
support for earycon debugging more widely available.

Cc: Florent Kermarrec <[e-mail]>
Cc: Mateusz Holenko <[e-mail]>
Cc: Joel Stanley <[e-mail]>
Cc: Gabriel L. Somlo <[e-mail]>
Reviewed-and-tested-by: Gabriel Somlo <[e-mail]>
Reviewed-by: Jiri Slaby <[e-mail]>
Reviewed-by: Joel Stanley <[e-mail]>
Signed-off-by: Stafford Horne <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			the driver will use only 32-bit accessors to read/write
 			the device registers.
 
+		liteuart,<addr>
+			Start an early console on a litex serial port at the
+			specified address. The serial port must already be
+			setup and configured. Options are not yet supported.
+
 		meson,<addr>
 			Start an early, polled-mode console on a meson serial
 			port at the specified address. The serial port must

drivers/tty/serial/Kconfig

···
 	bool "LiteUART serial port console support"
 	depends on SERIAL_LITEUART=y
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Say 'Y' or 'M' here if you wish to use the FPGA-based LiteUART serial
 	  controller from LiteX SoC builder as the system console

drivers/tty/serial/liteuart.c

···
 	return 0;
 }
 console_initcall(liteuart_console_init);
+
+static void early_liteuart_write(struct console *console, const char *s,
+				    unsigned int count)
+{
+	struct earlycon_device *device = console->data;
+	struct uart_port *port = &device->port;
+
+	uart_console_write(port, s, count, liteuart_putchar);
+}
+
+static int __init early_liteuart_setup(struct earlycon_device *device,
+				       const char *options)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	device->con->write = early_liteuart_write;
+	return 0;
+}
+
+OF_EARLYCON_DECLARE(liteuart, "litex,liteuart", early_liteuart_setup);
 #endif /* CONFIG_SERIAL_LITEUART_CONSOLE */
 
 static int __init liteuart_init(void)