ARM: EXYNOS: add code for setting/clearing boot flag

This code is needed for cpuidle (W-)AFTR mode support on Exynos3250.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>