gpu: ion: Add API to do cache operations

Clients need to be able to do cache operations
on ION buffers in the kernel. Add API to
flush, invalidate, or invalidate and flush the
cache of an ION buffer.

Change-Id: I2b676dbe32372b3c17e4aaf39f51878b105a699c
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>