cpu: Mark cpu_possible_mask as __ro_after_init

cpu_possible_mask is by definition "cpus which could be hotplugged without
reboot". It's a property which is fixed after kernel enumerates the
hardware configuration.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/41cd78af-92a3-4f23-8c7a-4316a04a66d8@p183