serial: altera_uart: add earlycon support

Nios2 currently uses its own early printk implementation, rather than
using unified earlycon support to show boot messages on altera_uart.

Add earlycon support to altera_uart so that other archs may use it.
Also, this (together with the corresponding patch for altera_jtaguart)
will allow the early printk implementation in arch/nios2 to be removed
in a future patch.

Cc: Ley Foon Tan <lftan@altera.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>