ARM: exynos_defconfig: Enable CONFIG_EXYNOS_IOMMU

EXYNOS_IOMMU is disabled in exynos_defconfig since it was known to cause
boot failures on Exynos Chromebooks. The recommendation was for IOMMU to
be enabled manually on systems as needed.

A recent exynos_drm change added a warning message when EXYNOS_IOMMU is
disabled. It is necessary to enable it to avoid the warning messages.
A few initial tests have shown that enabling EXYNOS_IOMMU might be safe
on Exynos Chrome-books.

Due to some other changes in the order of operations during boot
process, power domains are initialized very early and because of the
temporary lack of devices (which are not yet added to the system), are
turned off.

This practically stops FIMD for scanning framebuffer very early during
boot, before IOMMU gets initialized and "solves" the issue, which was
the reason to disable Exynos IOMMU by default on Exynos Snow Chromebooks.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
[krzk: Extend message with Marek Szyprowski's explanation]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>