acpi: Introduce acpi_arch_get_root_pointer() for getting rsdp address

Add an architecture specific function to get the address of the RSDP
table. Per default it will just return 0 indicating falling back to
the current mechanism.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: boris.ostrovsky@oracle.com
Cc: lenb@kernel.org
Cc: linux-acpi@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/20180219100906.14265-2-jgross@suse.com


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