ARM: sa1100: 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: Jochen Friedrich <jochen@scram.de>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>