ARM: EXYNOS: Add support for Exynos secure firmware

Some Exynos-based boards contain secure firmware and must use firmware
operations to set up some hardware.

This patch adds firmware operations for Exynos secure firmware and a way
for board code and device tree to specify that they must be used.

Example of use:

In board code:

  ...MACHINE_START(...)
          /* ... */
          .init_early   = exynos_firmware_init,
          /* ... */
  MACHINE_END

In device tree:

  / {
          /* ... */

          firmware@0203F000 {
                  compatible = "samsung,secure-firmware";
                  reg = <0x0203F000 0x1000>;
          };

          /* ... */
  };

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>