Boot arm64 kernel using booti command from U-boot.

Summary:
Boot arm64 kernel using booti command from U-boot. booti can relocate initrd
image into higher ram addresses, therefore align the initrd load address to 1GiB
and create VA = PA map for it. Create L2 pagetable entries to copy the initrd
image into KVA.
(parts of the code in https://reviews.freebsd.org/D13861 was referred and used
as appropriate)

Submitted by: Siddharth Tuli <siddharthtuli_gmail.com>
Reviewed by: manu
Sponsored by: Juniper Networks, Inc
Differential Revision: https://reviews.freebsd.org/D22255