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.comSigned-off-by: Ingo Molnar <mingo@kernel.org>