cpu/x86/early_reset: Mark assemblycode as 32bit

Allows to compile the file under x86_64 without errors.

The caller has to make sure to call the functions while in protected
mode, which is usually the case in early bootblock.

Change-Id: Ic6601e2af57e0acc6474fc3a4297e3d2281decd6
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48165
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/cpu/x86/early_reset.S

@@ -7,6 +7,7 @@
 
 #include <cpu/x86/mtrr.h>
 
+.code32
 .section .text
 .global check_mtrr