x86/apic: Mark acpi_mp_wake_* variables as __ro_after_init

acpi_mp_wake_mailbox_paddr and acpi_mp_wake_mailbox are initialized once during
ACPI MADT init and never changed.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kai Huang <kai.huang@intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Tao Liu <ltao@redhat.com>
Link: https://lore.kernel.org/r/20240614095904.1345461-3-kirill.shutemov@linux.intel.com