armv8r64-extras/zephyr: Add zephyr-philosophers sample application

The zephyr repository contains 3 "classic" sample applications which are
intended to run on all boards - hello_world, synchronization and
philosophers.

The first two are already supported in meta-armv8r64-extras, but there
was previously a defect related to use of the GIC in the Xen MPU branch
which prevented philosophers from working.

This has now been fixed (and the SRCREV updated in a previous patch), so
add a recipe for zephyr-philosophers.

zephyr-philosophers only supports running on a single-core, so force
ZEPHYR_NUM_CPUS to 1.

Issue-Id: SCM-4697
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I126eead7f43cfd4772634a0de7bf8f4f27179991