ARM: uniphier: change the external bus address mapping

In UniPhier SoCs before ProXstream2 and PH1-LD6b, two address spaces
 0x00000000 - 0x0fffffff
 0x40000000 - 0x4fffffff
are both mapped to the external bus (also called system bus),
so either was OK.

In the newest two SoCs, the former (0x00000000 - 0x0fffffff) is
assigned for the serial NOR interface.

Going forward, use the latter for the external bus.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>