ARM: socfpga: use of_iomap to map the SCU

Use of_iomap to map the "arm,cortex-a9-scu". By doing this, we can remove
map_io in socfpga.c.

Also, we can remove socfpga_smp_init_cpus, as arm_dt_init_cpu_maps is
already doing the CPU mapping.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>