The current fixup code is slightly wrong, and only works when the
root address cell number is one.

However, Armada XP has a root address cell number of two. In this case
we are currently applying the fixup on the child high base address,
while it should be applied on the child low base address.

Fix it and add some detailed explanation to avoid having to figure this
out each time.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>