ARM: EXYNOS: add exynos_set_boot_addr() helper

Add exynos_set_boot_addr() helper and covert existing code
(exynos_boot_secondary() and exynos_smp_prepare_cpus()) to
use it.

There should be no functional changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>