serial: altera_jtaguart: add earlycon support

Nios2 currently uses its own early printk implementation, rather than
using unified earlycon support to show boot messages on altera_jtaguart
(and altera_uart for that matter).

Add earlycon support to altera_jtaguart so that other archs may use it. Also,
this will allow the early printk implementation in arch/nios2 to eventually 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>