arch/x86: Declare global_smi_enable()

The call made at mp_ops.post_mp_init() generally
uses four different names. Unify these with followups.

  smm_southbridge_enable(SMI_EVENTS)
  smm_southbridge_enable_smi()
  hudson_enable_smi_generation()
  enable_smi_generation()

Furthermore, some platforms do not enable power button
SMI early. It may be preferred to delay the enablement,
but fow now provide global_smi_enable_no_pwrbtn() too.

Change-Id: I6a28883ff9c563289b0e8199cd2ceb9acd6bacda
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42355
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/include/cpu/x86/smm.h

@@ -38,6 +38,9 @@ int mainboard_io_trap_handler(int smif);
 
 void southbridge_smi_set_eos(void);
 
+void global_smi_enable(void);
+void global_smi_enable_no_pwrbtn(void);
+
 void cpu_smi_handler(void);
 void northbridge_smi_handler(void);
 void southbridge_smi_handler(void);