soc: qcom: ddr_stats: update driver to get reg in probe

This change optimizes to get reg during probe.
With this change no need to ioremap_nocache() reg for every
ddr_stats_show() call.

Change-Id: Ieb38280d1f03e1d613e638a7532b7b014525fbf1
Signed-off-by: Tushar Nimkar <tnimkar@codeaurora.org>
Signed-off-by: Minghao Zhang <minghao@codeaurora.org>