ARM: EXYNOS: Fix kernel panic when unplugging CPU1 on exynos

A look at the code reveals use of S5P_VA_SYSRAM macro, in case of certain SoC
revisions, which is not valid any longer, after SYSRAM started to be mapped
dynamically. The new dynamic mapping is stored in sysram_base_addr variable,
which is declared static in platsmp.c

This fix makes sysram_base_addr non-static, declared it in common.h and used
in pm.c instead of S5P_VA_SYSRAM.

Suggested-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>