armv8r64-extras/zephyr: Require COMPATIBLE_MACHINE definition

The Zephyr implemention has only been tested and validated on the
fvp-baser-aemv8r64 MACHINE, so set COMPATIBLE_MACHINE to "invalid" in
zephyr-base.inc and override in the machine-specific include.

Issue-Id: SCM-4386
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Idcbb1b83a3944b14c0692546892f162b4e77f778