ACPI / BGRT: Don't let users configure BGRT on non X86 systems

Fengguang Wu's 0-Day kernel build testing backend found the
following build error for an allmodconfig build on ia64:

   drivers/built-in.o: In function `show_yoffset':
>> bgrt.c:(.text+0xe5a71): undefined reference to `bgrt_tab'
>> bgrt.c:(.text+0xe5a91): undefined reference to `bgrt_tab'
   drivers/built-in.o: In function `show_xoffset':
>> bgrt.c:(.text+0xe5b51): undefined reference to `bgrt_tab'
>> bgrt.c:(.text+0xe5b71): undefined reference to `bgrt_tab'
   drivers/built-in.o: In function `show_type':
>> bgrt.c:(.text+0xe5c31): undefined reference to `bgrt_tab'
   drivers/built-in.o:bgrt.c:(.text+0xe5c51): more undefined references to `bgrt_tab' follow
   drivers/built-in.o: In function `bgrt_init':
   bgrt.c:(.init.text+0x8931): undefined reference to `bgrt_image'
   bgrt.c:(.init.text+0x8932): undefined reference to `bgrt_image_size'
   bgrt.c:(.init.text+0x8950): undefined reference to `bgrt_image'
   bgrt.c:(.init.text+0x8960): undefined reference to `bgrt_image_size'

The problem is that all these undefined names are provided by
arch/x86/platform/efi/efi-bgrt.c - which is obviously not available
to the ia64 build.

It doesn't seem useful to provide the BGRT support for Itanium
(many systems are headless and have no graphics at all). So
just don't let users configure this driver on non-X86 machines.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>