nios2: enable earlycon support

Enable generic earlycon support for nios2. This e.g. allows to use a
8250/16650 UART as earlycon.

In order to get the earlycon, we just need to call parse_early_param()
in early_init_devtree() as soon as the device tree is initially scanned.
By adding an stdout-path property to the dts (done in this patch for
10m50_devboard), the earlycon can be used.

In order to provide early printk support, we need to provide a dummy
implementation of early_console_write(), so that
arch/nios2/kernel/early_printk.c can still be compiled if neither
SERIAL_ALTERA_JTAGUART_CONSOLE nor SERIAL_ALTERA_UART_CONSOLE is
selected. As soon as the altera_uart and altera_jtaguart support
earlycon, the entire file can be removed.
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>

arch/nios2/Kconfig.debug

@@ -18,7 +18,6 @@ config EARLY_PRINTK
 	bool "Activate early kernel debugging"
 	default y
 	select SERIAL_CORE_CONSOLE
-	depends on SERIAL_ALTERA_JTAGUART_CONSOLE || SERIAL_ALTERA_UART_CONSOLE
 	help
 	  Enable early printk on console
 	  This is useful for kernel debugging when your machine crashes very

arch/nios2/boot/dts/10m50_devboard.dts

@@ -244,6 +244,7 @@ led_fpga3: fpga3 {
 	};
 
 	chosen {
-		bootargs = "debug console=ttyS0,115200";
+		bootargs = "debug earlycon console=ttyS0,115200";
+		stdout-path = &a_16550_uart_0;
 	};
 };

arch/nios2/kernel/early_printk.c

@@ -81,8 +81,11 @@ static void early_console_write(struct console *con, const char *s, unsigned n)
 }
 
 #else
-# error Neither SERIAL_ALTERA_JTAGUART_CONSOLE nor SERIAL_ALTERA_UART_CONSOLE \
-selected
+
+static void early_console_write(struct console *con, const char *s, unsigned n)
+{
+}
+
 #endif
 
 static struct console early_console_prom = {

arch/nios2/kernel/setup.c

@@ -137,6 +137,8 @@ asmlinkage void __init nios2_boot_init(unsigned r4, unsigned r5, unsigned r6,
 		strncpy(boot_command_line, CONFIG_CMDLINE, COMMAND_LINE_SIZE);
 #endif
 #endif
+
+	parse_early_param();
 }
 
 void __init setup_arch(char **cmdline_p)