altera_uart: Fix missing prototype for registering an early console

Simply add an early_altera_uart_setup() prototype declaration, otherwise
platform code have to do it in .c files, which is ugly.

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
Acked-by: Tobias Klauser <tklauser@distanz.ch>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>