powerpc/mpic: handle subsys_system_register() failure

mpic_init_sys() currently doesn't check whether
subsys_system_register() succeeded or not. Check the return code of
subsys_system_register() and clean up if there's an error.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>