powerpc/mpc83xx: Add the missing set_freezable() for agent_thread_fn()

powerpc/mpc83xx: Add the missing set_freezable() for agent_thread_fn()

The kernel thread function agent_thread_fn() invokes the try_to_freeze()
in its loop. But all the kernel threads are non-freezable by default.
So if we want to make a kernel thread to be freezable, we have to invoke
set_freezable() explicitly.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231221044510.1802429-2-haokexin@gmail.com