arm64: Add SMC and HVC commands

This patch adds smc and hvc commands, that allow issuing Secure Monitor
Calls and Hypervisor Calls conforming to the ARM SMC Calling Convention.

Add Kconfig items to allow each command can be individually enabled.

Signed-off-by: Michalis Pappas <mpappas@fastmail.fm>
Reviewed-by: Simon Glass <sjg@chromium.org>