ARM: iop13xx: use __iomem pointers for MMIO

ARM is moving to stricter checks on readl/write functions,
so we need to use the correct types everywhere.

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>