early_printk: Allow more than one early console

It is desirable to be able to use one early boot device to debug
another or to have multiple places you can see the early boot
diagnostics, such as the vga screen or serial device.

This patch changes the early_printk console device registration to
allow more than one early printk device to get registered via
register_console().

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>