ARM: EXYNOS: add exynos_get_boot_addr() helper

Add get_cpu_boot_addr() firmware operation and then
exynos_get_boot_addr() helper.

This is a preparation for adding coupled cpuidle support
for Exynos3250 SoC.

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>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>