ARM: Add save_boot_params for ARMv8

Implement a hook to allow boards to save boot-time CPU state for later
use. When U-Boot is chain-loaded by another bootloader, CPU registers may
contain useful information such as system configuration information. This
feature mirrors the equivalent ARMv7 feature.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>