ARM: at91/dt: add specific DT soc init

This will allow to have static Device mapping and DT probe mapping for the
System Controller.

Temporary keep the call to ioremap_registers() until we have the binding
for the SDRAM/DDR Controller.

Temporary keep the main clock hardcoded to 12MHz until we have the binding
for the PMC.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>