msm: ion: Export msm_ion_do_cache_offset_op to do cache operations

The current API exposed to clients for doing
cache operations didn't provide option to
specify offset within ion buffer and goes
and performs operation on complete buffer.
Export new function for clients which wants to
perform cache operation on specific range
within ion buffer.

We would also need to amend the current generic
ion_do_cache_op function to correctly take into
account the offset within buffer.

Change-Id: I5e4e7beda47cbbd43783048c64fe5adb2beb7023
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>