arm64: start defining a std.apple configuration

The M1 uses FDT, and has bge to start with. Add a SOC_* option for
the first SoC we'll be supporting.

IOMMU is added commented out because it does have it, but IOMMU is not
well-tested on aarch64. An initial version of the DART driver will be
upstreamed that just puts the DARTs that support bypass mode into bypass
mode -- we'll be missing some functionality, but we at least still end
up with some USB ports.

Reviewed by: karels, manu
Input from: jrtc27 (IOMMU)
Differential Revision: https://reviews.freebsd.org/D39823