arm64: defconfig: Remove unnecessary configs in genericarmv8-64

Certain configs that are enabled in genericarmv8-64 are not
required for the usecases that the kernel is supposed to cater.
For example, IOMMU support is not required for genericarmv8-64,
yet, the drivers for it are built into the kernel image. This
leads to a bloated kernel image with a larger memory footprint,
as well as larger runtime memory footprint, as certain drivers
may allocate data structures that go by unused. Remove certain
extraneous configs.

Change-Id: Iefe8dd2a35a4a29ed97338ec5fea5c147da14eda
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>