thunderbolt: Compile on x86 only

So far Thunderbolt is (unfortunately) an Intel proprietary technology
that is only available on x86, so compiling on other arches is pointless
except for testing purposes.  Amend Kconfig accordingly.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andreas Noever <andreas.noever@gmail.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/7dfda728d3ee8a33c80c49b224da7359c6015eea.1479456179.git.lukas@wunner.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/thunderbolt/Kconfig

@@ -1,6 +1,7 @@
 menuconfig THUNDERBOLT
 	tristate "Thunderbolt support for Apple devices"
 	depends on PCI
+	depends on X86 || COMPILE_TEST
 	select APPLE_PROPERTIES if EFI_STUB && X86
 	select CRC32
 	help