soc: qcom: ramdump: Use dma_remap/dma_unremap for ramdump read

The memory allocated from the general cma heap cannot be remapped
using ioremap. This problem was encountered on msm8909 when trying
to dump the venus memory.

Hence, handle the ramdump_read mapping using dma_remap/dma_unremap
instead of ioremap_nocache/iounmap.

Change-Id: Ia161851578c2e27f9364cf7c229bc5fd40e43d24
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>