ARM: at91: add ioremap_registers entry point to soc setup

this will allow to ioremap the register of the PIT, PMC and others
and make the code soc independent

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