[kernel][arm64] Remove remaining use of MDI from the arm64 kernel

Kernel drivers are now configured via bootdata passed from the boot shims.

Change-Id: I056b9b2c74a99226d98416b0308480b85460e918

[hikey960] Implement dm reboot and dm reboot-bootloader

Hikey can now reboot itself and also boot into the bootloader
Unfortunately I haven't figured out how to implement shutdown yet.

ZX-1506 #done
ZX-1507 #done

Change-Id: Id94a1a433140c19bbfaec5ed4b342f8514f67fe5