x86/ACPI/sleep: Move acpi_get_wakeup_address() into sleep.c, remove <asm/realmode.h> from <asm/acpi.h>

Move the definition of acpi_get_wakeup_address() into sleep.c to break
linux/acpi.h's dependency (by way of asm/acpi.h) on asm/realmode.h.
Everyone and their mother includes linux/acpi.h, i.e. modifying
realmode.h results in a full kernel rebuild, which makes the already
inscrutable real mode boot code even more difficult to understand and is
positively rage inducing when trying to make changes to x86's boot flow.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Link: https://lkml.kernel.org/r/20191126165417.22423-13-sean.j.christopherson@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/acpi/sleep.c

@@ -26,6 +26,17 @@ unsigned long acpi_realmode_flags;
 static char temp_stack[4096];
 #endif
 
+/**
+ * acpi_get_wakeup_address - provide physical address for S3 wakeup
+ *
+ * Returns the physical address where the kernel should be resumed after the
+ * system awakes from S3, e.g. for programming into the firmware waking vector.
+ */
+unsigned long acpi_get_wakeup_address(void)
+{
+	return ((unsigned long)(real_mode_header->wakeup_start));
+}
+
 /**
  * x86_acpi_enter_sleep_state - enter sleep state
  * @state: Sleep state to enter.