ARM: EXYNOS: Remove exynos_subsys registration

'exynos_subsys' has no users. Remove this code.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>