src: Replace common MSR addresses with macros

Change-Id: I9fba67be12483ea5e12ccd34c648735d409bc8b0
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/29252
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Richard Spiegel <richard.spiegel@silverbackltd.com>

src/cpu/amd/microcode/microcode.c

@@ -127,7 +127,7 @@ static void apply_microcode_patch(const struct microcode *m)
 	UCODE_DEBUG("patch id to apply = 0x%08x\n", m->patch_id);
 
 	/* read the patch_id again */
-	msr = rdmsr(0x8b);
+	msr = rdmsr(IA32_BIOS_SIGN_ID);
 	new_patch_id = msr.lo;
 
 	UCODE_DEBUG("updated to patch id = 0x%08x %s\n", new_patch_id,