ARM: ixp4xx: use runtime ioremap hook

Convert ixp4xx platforms to use run-time ioremap hook instead of the
compile time hook.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>