serial: 8250: pxa: add devicetree earlyconsole

Transfer the device-tree pxa uart handling from 8250_of to the new
8250_pxa.  As a corollary, add the early console definition into
8250_pxa.

This enables to have the same uart node for the early console and the
normal uart.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>