ACPI/sleep: Convert acpi_wakeup_address into a function

Convert acpi_wakeup_address from a raw variable into a function so that
x86 can wrap its dereference of the real mode boot header in a function
instead of broadcasting it to the world via a #define.  This sets the
stage for a future patch to move x86's definition of the new function,
acpi_get_wakeup_address(), out of asm/acpi.h and thus break acpi.h's
dependency on asm/realmode.h.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Link: https://lkml.kernel.org/r/20191126165417.22423-12-sean.j.christopherson@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>