ion: implement interface to export total_pools_kb

The ABI interface "/sys/kernel/ion/total_pools_kb"
was declared but not implemented before in Qcom system heap.
Implement it now.

Change-Id: I3140863c10600166d89ed055ba3be8f368d7e92a
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>