soc: qcom: minidump: Change the way to locate log_buf

Use the exported function instead of kallsyms_lookup to get
log_buf location, this can save a lot of time during boot up.

Change-Id: I9ec242ad6edfc3ba101198a5c1ed15bb5ad085a0
Signed-off-by: Huang Yiwei <hyiwei@codeaurora.org>