iommu/exynos: Add iommu driver for EXYNOS Platforms

This is the System MMU driver and IOMMU API implementation for
EXYNOS SoC platforms. EXYNOS platforms has more than 10 System
MMUs dedicated for each multimedia accelerators.

The System MMU driver is already in arc/arm/plat-s5p but it is
moved to drivers/iommu due to Ohad Ben-Cohen gathered IOMMU
drivers there.

Any device driver in EXYNOS platforms that needs to control its
System MMU must call platform_set_sysmmu() to inform System MMU
driver who will control it. platform_set_sysmmu() is defined in
<mach/sysmmu.h>

Signed-off-by: KyongHo Cho <pullip.cho@samsung.com>
Acked-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>