platform: Check for ARCH_EXYNOS separately

ARCH_EXYNOS is going to be excluded from PLAT_S5P, so it must be checked
separately in Exynos-related Kconfig entries.

Cc: linux-media@vger.kernel.org
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>
Cc: Kamil Debski <k.debski@samsung.com>
Cc: Tomasz Stanislawski <t.stanislaws@samsung.com>
Cc: Jeongtae Park <jtp.park@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>