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>