ion: Implement support for partial cache maintenance

In order to improve performance allow ION clients the ability
to apply cache maintenance to only a subset of an ION buffer.

Kernel clients will be able to use dma_buf_begin_cpu_access_partial
and dma_buf_end_cpu_access_partial functions to only apply cache
maintenance to a range within the ION buffer.

Userspace clients will be able to restrict cache maintenance to only
the subset of the ION buffer which is mmap(ed) by setting the
DMA_BUF_SYNC_USER_MAPPED flag when calling the DMA_BUF_IOCTL_SYNC IOCT.

Change-Id: I1f0a3f658d78e59fe30becf42e7086f60779a715
Signed-off-by: Liam Mark <lmark@codeaurora.org>