sunxi: add support for UART2 on A23/A33

Add support for UART2 (2-pin version but note that RTS/CTS pins are available
pn that port for possible future use), can be selected in config
by using CONFIG_CONS_INDEX=3

Signed-off-by: Laurent Itti <laurentitti@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>