hwaccess: fix build on non-x86 targets

The changes to hwaccess in commit 49d758698a
cause build failure on non-x86 systems because the hwaccess_x86_*
headers are included in some files that are built for all platforms
(particularly those in the internal programmer) and those headers in
turn include <sys/io.h> which only exists on x86.

This change avoids including those headers on non-x86 platforms so
the internal programmer can be built without errors.

The comment on the stub implementation of rget_io_perms() is also
modified to remove references to non-x86 platforms, since that file is
only built on x86 now.

BUG=None
TEST=meson build succeeds for both x86 and ARM targets

Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Change-Id: I20f122679c30340b2c73afd7419e79644ddc3c4e
Reviewed-on: https://review.coreboot.org/c/flashrom/+/61194
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

board_enable.c

@@ -25,11 +25,13 @@
 #include <stdlib.h>
 #include "flash.h"
 #include "programmer.h"
-#include "hwaccess_x86_io.h"
-#include "hwaccess_x86_msr.h"
 #include "platform/pci.h"
 
 #if defined(__i386__) || defined(__x86_64__)
+
+#include "hwaccess_x86_io.h"
+#include "hwaccess_x86_msr.h"
+
 /*
  * Helper functions for many Winbond Super I/Os of the W836xx range.
  */