soc: qcom: ddr_stats: API to read ddr residency

DDR residency can be read using sysfs node.
This change provides an API to get ddr residency so that it
can be read from any kernel drivers.

Change-Id: I6f0d508994a1f5e427e2cbce6f0229013fec387f
Signed-off-by: Minghao Zhang <minghao@codeaurora.org>