soc/intel/common,skl: set MSR LT_LOCK_MEMORY once, not per thread

The MSR LT_LOCK_MEMORY is package-scoped, not thread-scoped. Only set it
once.

Tested on Acer ES1-572 by checking chipsec results.

Change-Id: If3d61fcbc9ab99b6c1b7b74881e6d9c6be04a498
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44242
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/soc/intel/skylake/finalize.c

@@ -84,8 +84,7 @@ static void soc_lockdown(struct device *dev)
 		pci_write_config8(dev, GEN_PMCON_A, reg8);
 	}
 
-	/* Lock chipset memory registers to protect SMM */
-	mp_run_on_all_cpus(cpu_lt_lock_memory, NULL);
+	cpu_lt_lock_memory();
 }
 
 static void soc_finalize(void *unused)