iio: adc: Fix exynos_adc dependencies

EXYNOS_ADC shouldn't only depend on OF. It is useless on architectures
other than Exynos so it should depend on ARCH_EXYNOS (which implies
OF.)

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>