arch/x86/post.c: Hide cmos_post_code from SMM context

Code in SMM segment using cmos_post_code will give compiler error since
cmos_post_code function is not getting compiled during SMM stage.

Also as per patch discussion, CMOS uses a split IO transaction and it's not
really safe to call cmos_post_code from SMM context. Thus we'll hide the
call for SMM context.

Change-Id: Iffdcccaad48e7ad96e068d07046630fbe4297e65
Signed-off-by: Maulik V Vaghela <maulik.v.vaghela@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38370
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/arch/x86/post.c

@@ -21,6 +21,6 @@ void arch_post_code(uint8_t value)
 	if (CONFIG(POST_IO))
 		outb(value, CONFIG_POST_IO_PORT);
 
-	if (CONFIG(CMOS_POST))
+	if (CONFIG(CMOS_POST) && !ENV_SMM)
 		cmos_post_code(value);
 }