powerpc: use machine_subsys_initcall() for opal_hmi_handler_init()

As opal_message_init() uses machine_early_initcall(powernv, ), and
opal_hmi_handler_init() depends on that early initcall, so it also needs
use machine_* to check the machine_id.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>