x86/olpc: Indicate that legacy PC XO-1 platform should not register RTC

On OLPC XO-1, the RTC is discovered via device tree from the arch
initcall. Don't let the PC platform register another one from its device
initcall, it's not going to work:

  sysfs: cannot create duplicate filename '/devices/platform/rtc_cmos'
  CPU: 0 PID: 1 Comm: swapper Not tainted 4.19.0-rc6 #12
  Hardware name: OLPC XO/XO, BIOS OLPC Ver 1.00.01 06/11/2014
  Call Trace:
   dump_stack+0x16/0x18
   sysfs_warn_dup+0x46/0x58
   sysfs_create_dir_ns+0x76/0x9b
   kobject_add_internal+0xed/0x209
   ? __schedule+0x3fa/0x447
   kobject_add+0x5b/0x66
   device_add+0x298/0x535
   ? insert_resource_conflict+0x2a/0x3e
   platform_device_add+0x14d/0x192
   ? io_delay_init+0x19/0x19
   platform_device_register+0x1c/0x1f
   add_rtc_cmos+0x16/0x31
   do_one_initcall+0x78/0x14a
   ? do_early_param+0x75/0x75
   kernel_init_freeable+0x152/0x1e0
   ? rest_init+0xa2/0xa2
   kernel_init+0x8/0xd5
   ret_from_fork+0x2e/0x38
  kobject_add_internal failed for rtc_cmos with -EEXIST, don't try to
    register things with the same name in the same directory.
  platform rtc_cmos: registered platform RTC device (no PNP device found)
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Ingo Molnar <mingo@redhat.com>
CC: x86-ml <x86@kernel.org>
Link: http://lkml.kernel.org/r/20181004160808.307738-1-lkundrak@v3.sk

arch/x86/platform/olpc/olpc-xo1-rtc.c

@@ -16,6 +16,7 @@
 
 #include <asm/msr.h>
 #include <asm/olpc.h>
+#include <asm/x86_init.h>
 
 static void rtc_wake_on(struct device *dev)
 {
@@ -75,6 +76,8 @@ static int __init xo1_rtc_init(void)
 	if (r)
 		return r;
 
+	x86_platform.legacy.rtc = 0;
+
 	device_init_wakeup(&xo1_rtc_device.dev, 1);
 	return 0;
 }