ARM: EXYNOS: Change System MMU platform device definitions

Handling System MMUs with an identifier is not flexible to manage
System MMU platform devices because of the following reasons:
1. A device driver which needs to handle System MMU must know the ID.
2. A System MMU may not present in some implementations of Exynos family.
3. Handling System MMU with IOMMU API does not require an ID.

This patch is the result of removing ID of System MMUs.
Instead, a device driver that needs to handle its System MMU must
use IOMMU API while its descriptor of platform device is given.

This patch also includes the following enhancements:
- A System MMU device becomes a child if its power domain device.
- clkdev

Signed-off-by: KyongHo Cho <pullip.cho@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>