xen: add per-platform defaults for NR_CPUS

xen: add per-platform defaults for NR_CPUS

Add specific per-platform defaults for NR_CPUS. Note that the order of
the defaults matter: they need to go first, otherwise the generic
defaults will be applied.

This is done so that Xen builds customized for a specific hardware
platform can have the right NR_CPUS number.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
CC: JBeulich@suse.com
CC: andrew.cooper3@citrix.com
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Changes in v6:
- remove useless additional default for ALL