cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

In the Linux kernel, the following vulnerability has been resolved:

cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

On arm64, when booting with `maxcpus` greater than the number of present
CPUs (e.g., QEMU -smp cpus=4,maxcpus=8), some CPUs are marked as 'present'
but have not yet been registered via register_cpu(). Consequently,
the per-cpu device objects for these CPUs are not yet initialized.

In cpuhp_smt_enable(), the code iterates over all present CPUs. Calling
_cpu_up() for these unregistered CPUs eventually leads to
sysfs_create_group() being called with a NULL kobject (or a kobject
without a directory), triggering the following warning in
fs/sysfs/group.c:

WARNING: fs/sysfs/group.c:137 at internal_create_group+0x41c/0x4bc, CPU#2: sh/181
[...]
Call trace:
internal_create_group+0x41c/0x4bc (P)
sysfs_create_group+0x18/0x24
topology_add_dev+0x1c/0x28
cpuhp_invoke_callback+0x104/0x20c
__cpuhp_invoke_callback_range+0x94/0x11c
_cpu_up+0x200/0x37c

When booting with ACPI, arm64 smp_prepare_cpus() currently sets all
enumerated CPUs as "present" regardless of their status in the MADT. This
causes issues with SMT hotplug control. For instance, with QEMU's
"-smp 4,maxcpus=8" configuration, the MADT GICC entries are populated as
follows:

1. The first four CPUs: `Enabled` set but `Online Capable` not set.

2. The remaining four CPUs: `Online Capable` set but `Enabled` not set
to support potential hot-plugging.

Fix this by:

1. When booting with ACPI, checking the ACPI_MADT_ENABLED flag in the GICC
entry before calling set_cpu_present() during SMP initialization.

2. Properly managing the present mask in acpi_map_cpu() and
acpi_unmap_cpu() to support actual CPU hotplug events, This aligns with
other architectures like x86 and LoongArch.

3. Update the arm64 CPU hotplug documentation to no longer state that all
online-capable vCPUs are marked as present by the kernel at boot time.

This ensures that only physically available or explicitly enabled CPUs
are in the present mask, keeping the SMT control logic consistent with
the actual hardware state.