sh: Use ioremap_fixmed to map the SM501 DRAM config register

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: Matt Fleming <matt@console-pimps.org>