When retrieving a phandle for a CPU node using "fdtget -t x", the
resulting hex number (in case the phandle exists) lacks the '0x' prefix.
This value is subsequently used to construct the cpupool-cpus property.
This results in an incorrect property generation that leads to Xen
failure (example: cpupool-cpus < a> instead of < 0xa>). Address the
issue by adding 0x prefix to the result from fdtget.

Additionally, avoid unnecessary regeneration of the phandle if it already
exists.

Fixes: b687773b

 ("Add support for Xen boot-time cpupools")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>