ARM: iop32x: 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: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>