Revert "ARM: exynos: enable multiplatform support"

This reverts commit bd51de53e1be9896d815bbea30560262216d4616.

Turns out that multiplatform breaks some uses cases, such as when you
have an existing defconfig, since it adds the new EXYNOS_SINGLE config
option as a dependecy. As a result, nearly all exynos config options
will be disabled by default.

Reverting instead of rebasing since this branch is pulled in as a
dependency elsewhere.

Signed-off-by: Olof Johansson <olof@lixom.net>