ARM: EXYNOS: mcpm rename the power_down_finish

Since commit 166aaf39 ("ARM: 8029/1: mcpm: Rename the power_down_finish()
functions to be less confusing") changed the name of power_down_finish to
wait_for_cpu_powerdown, so use new member name wait_for_cpu_powerdown.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>