arm64: Add per SoC family kernel config

There is multiple reason for this :
- This makes it easier to see which driver is needed for each SoC
- This makes it easier to create a custom config for one

arm64: Add per SoC family kernel config

There is multiple reason for this :
- This makes it easier to see which driver is needed for each SoC
- This makes it easier to create a custom config for one SoC
- This really reduce boot time (which some people might want)

Some explaination about the files :
- std.arm64 contains all standard kernel option
- std.dev contains all the standard kernel devices
- std.<soc> contains all drivers needed to boot on this SoC family
- <SOC> includes std.arm64, std.dev and std.<soc>
- GENERIC includes std.arm64, std.dev and all std.<soc>

Sponsored by: Diablotin Systems
MFC After: 2 months
Reviewed by: mmel, cognet, imp
Differential Revision: https://reviews.freebsd.org/D30474

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