[POWERPC] 4xx: Split early debug output and early boot console for 44x

Currently there's no way to enable early boot console on PowerPC 44x
not specifying uart's physical address in kernel config, which is used
for very early debug messages. This patch splits very early debug output
(which needs uart physical address in kernel config) and early boot console
(which searches for uarts in the device tree using find_legacy_serial_ports).
We enable early boot console for all 44x processors, while (dangerous)
early debug is user-selectable.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>