ANDROID: arm64: gki_defconfig: Enable ARCH_EXYNOS

Just like the presently enabled ARCH_QCOM, ARCH_SUNXI and ARCH_HISI
symbols, ARCH_EXYNOS based platforms require drivers which currently
depend on the symbol being enabled.

A recent attempt to remove one of these dependencies was firmly NACKed
by a bunch of upstream Maintainers [0], with one of them stating that
ARCH_EXYNOS should be enabled by any generic kernel which planned on
supporting the associated platforms.

Once this has been applied, we can revert the associated hacks we're
currently carrying in android-mainline.

[0] https://lore.kernel.org/lkml/20200220102628.3371996-1-gregkh@linuxfoundation.org/



Bug: 142159113
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I1277f173c1a39c1cdd23bca27ee5f0deb784e3fb