lib/acpi: Fix the behavior to fit with how it used to be

z_acpi_get_cpu() used to retrieve the local apic on enabled CPU, where
n was about the n'th enabled CPU, not just the n'th local apic.
The system indeed keeps local apic info also about non-enabled CPU,
and we don't care about these as there is nothing to do about it.

This issue exists on up_squared board for instance, but it's a common
one anyway.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
(cherry picked from commit c294b7d2)