exynos5: add trace feature #ifdef in exynos5-common.h

We can enable / disable trace feature from the FTRACE config options.
To enable, compile U-Boot with FTRACE=1.

This patch add #ifdef FTRACE in exynos5-common.h for enable/disable
to use FTRACE configs instead of having to change board config files.

Signed-off-by: Inha Song <ideal.song@samsung.com>
Acked-by: Simon Glass <sjg@chroimum.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>