AArch64: Add direct kernel boot support

[jongwu]

trafficstars

Direct kernel boot, that load and start kernel specified by kernel cmdline, is an important feature in some scenairo like secure containers, confidential container.

How does it work?

1. Specify kernel image and firmware from VM cmdline; (user)
2. Load kernel image somewhere inside memory and store kernel info (including kernel location and size) inside FDT/Chosen; (VMM does)
3. When firmware boot, retrive kernel info from FDT/Chosen; (RF)
4. Load kernel to kernel start address; (RF)
5. Prepare kernel cmdline;(RF)
6. Start kernel;(RF)

Dependency: CH lacks of support for this feature;