ARM64: Kconfig: Ensure Kconfig option is selectable

The ARM64_DMA_USE_IOMMU option was not selectable from the menuconfig
gui since it was not listed under a menu. Fix this.

Change-Id: Ieabc85fde23da8b2a7e32326f6c5fe3f48bc8a72
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>