xen/arm: domain_build: harden make_cpus_node()

xen/arm: domain_build: harden make_cpus_node()

make_cpus_node() is using a static buffer to generate the FDT node name.
While mpdir_aff is a 64-bit integer, we only ever use the bits [23:0] as
only AFF{0, 1, 2} are supported for now.

To avoid any potential issues in the future, check that mpdir_aff has
only bits [23:0] set.

Take the opportunity to reduce the size of the buffer. Indeed, only 8
characters are needed to print a 32-bit hexadecimal number. So
sizeof("cpu@") + 8 + 1 (for '\0') = 13 characters is sufficient.

Fixes: c81a791d34 (xen/arm: Set 'reg' of cpu node for dom0 to match MPIDR's affinity)
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Juergen Gross <jgross@suse.com>