ion: Add Kconfig dependency to ARM

The ion code has some very specific arm-isms which keeps it
from building on other architectures. These should probably be
resolved, but in the mean time, add a dependency on CONFIG_ARM
to avoid build failures.

v2: Fix earlier flub, sending out an early untested version of
the patch.

Change-Id: I5979af1ad59d1eeddd9e08763b1cbc946cf82339
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Rebecca Schultz Zavin <rebecca@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>