src/mb: Shorten 'include <arch/x86/include/arch/acpigen.h>'

'include <arch/acpigen.h>' is good enough.

Change-Id: Idc96376571715f5dd2c386f187b5c6d1613accee
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31779
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/getac/p470/cstates.c

@@ -12,7 +12,7 @@
  */
 
 #include <device/device.h>
-#include <arch/x86/include/arch/acpigen.h>
+#include <arch/acpigen.h>
 #include <southbridge/intel/i82801gx/i82801gx.h>
 
 static acpi_cstate_t cst_entries[] = {