ARM: EXYNOS: add secure firmware support to AFTR mode code

* Move cp15 registers saving to exynos_save_cp15() helper and add
  additional helper usage to do_idle firmware method.

* Use resume firmware method instead of exynos_cpu_restore_register()
  and skip exynos_cpu_save_register() on boards with secure firmware
  enabled.

* Use sysram_ns_base_addr + 0x24/0x20 addresses instead of the default
  ones used by exynos_cpu_set_boot_vector() on boards with secure
  firmware enabled.

* Use do_idle firmware method instead of cpu_do_idle() on boards with
  secure firmware enabled.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>