nb/intel/nehalem: Call smm_region_start() function

This also removes the unnecessary mask.

TEST: X201 Boots again.

Change-Id: Ia637bd01cd7dc1aecd1a87a739d5243c70419553
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33046
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/northbridge/intel/nehalem/ram_calc.c

@@ -35,7 +35,7 @@ static uintptr_t smm_region_start(void)
 
 u32 northbridge_get_tseg_base(void)
 {
-	return (u32)smm_region_start & ~1;
+	return (u32)smm_region_start();
 }
 
 void *cbmem_top(void)