soc: mem_dump: fix physical address mapping issue

Using phys_to_virt can't get correct virtual address. Because the physical
address is continuous, the virtual address can be obtained by using the
offset of the physical address.

Change-Id: Ifdf57820273b2466dca0e486b42ed6895bff0f48
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>