exynos: Enable config_distro_defaults.h

exynos: Enable config_distro_defaults.h

...and remove explicit setting of things which this implies. This is done for
all exynos platforms (4 & 5) so it is added to exynos-common.h

I'm mainly interested in CONFIG_CMD_BOOTZ and CONFIG_SUPPORT_RAW_INITRD

I have build tested on all exynos platforms (MAKEALL -s exynos), but only boot
tested on arndale.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Guillaume GARDET <guillaume.gardet@free.fr>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>