parisc: Prevent panic at system halt

When issuing a "shutdown -h now", the reboot syscall calls kernel_halt()
which shouldn't return, otherwise one gets this panic:

reboot: System halted
Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000000
CPU: 0 PID: 1 Comm: systemd-shutdow Not tainted 4.16.0-32bit+ #560
Backtrace:
 [<1018a694>] show_stack+0x18/0x28
 [<106e68a8>] dump_stack+0x80/0x10c
 [<101a4df8>] panic+0xfc/0x290
 [<101a90b8>] do_exit+0x73c/0x914
 [<101c7e38>] SyS_reboot+0x190/0x1d4
 [<1017e444>] syscall_exit+0x0/0x14

Fix it by letting machine_halt() call machine_power_off() which doesn't
return.

Signed-off-by: Helge Deller <deller@gmx.de>