xen/arm64: head: Correctly report the HW CPU ID

xen/arm64: head: Correctly report the HW CPU ID

There are no reason to consider the HW CPU ID will be 0 when the
processor is part of a uniprocessor system. At best, this will result to
conflicting output as the rest of Xen use the value directly read from
MPIDR_EL1.

So remove the zeroing and logic to check if the CPU is part of a
uniprocessor system.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>