ARM: EXYNOS: Use wfi macro in platform_do_lowpower

This patch is originally based on commit b3377d18 ("ARM: 7064/1:
vexpress: Use wfi macro in platform_do_lowpower.")

Current Exynos CPU hotplug code includes a hardcoded WFI instruction,
in ARM encoding. When the kernel is compiled in Thumb-2 mode, this
is invalid and causes the machine to hang hard when a CPU is offlined.

Use wfi macro instead of the hardcoded WFI instruction.

Signed-off-by: Leela Krishna Amudala <leela.krishna@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>