xen/arm: acpi: Fix SMP support when booting with ACPI

xen/arm: acpi: Fix SMP support when booting with ACPI

The variable enabled_cpus is used to know the number of CPU enabled in
the MADT.

Currently this variable is used to check the validity of the boot CPU.
It will be considered invalid when "enabled_cpus > 1".

However, this condition also means that multiple CPUs are present on the
system. So secondary will never be brought up.

The correct way to check the validity of the boot CPU is to use the
variable bootcpu_valid.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>