staging: ion: Add X86 dependency for ION_POOL_CACHE_POLICY

ION_POOL_CACHE_POLICY uses x86 specific commands.
Only allow it to be used for x86.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Change-Id: Ic66e175fa41486b0b5f77872f2193add77cbe66f
(cherry picked from commit 43f4056cbec017c4414d74b032e9c14e009eabb0)