arm64: EXYNOS: Consolidate ARCH_EXYNOS7 symbol into ARCH_EXYNOS

The ARMv8 Exynos family SoCs in Linux kernel are currently:
 - Exynos5433 (controlled by ARCH_EXYNOS),
 - Exynos7 (controlled by ARCH_EXYNOS7).

It duplicates Kconfig symbols unnecessarily, so consolidate them into
one ARCH_EXYNOS. Future SoCs could fall also under the ARCH_EXYNOS
symbol.

The commit should not bring any visible functional change.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>