ARM: spear13xx: 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: Viresh Kumar <viresh.linux@gmail.com>
Cc: Shiraz Hashim <shiraz.hashim@st.com>
Cc: spear-devel@list.st.com
Signed-off-by: Arnd Bergmann <arnd@arndb.de>