We need to write to the DRAM config register very early and at such an
early stage ioremap() is not available. So use ioremap_fixed() to map
the register.

The reason that we are avoiding using the legacy P2 mapping is that
there will come a day when the legacy P2 mappings no longer exist.
Signed-off-by: NMatt Fleming <matt@console-pimps.org>