dmaengine/dw_dmac fix: use readl & writel instead of __raw_readl & __raw_writel

On ARMv7 cores, device memory mapped as Normal Non-cacheable, may not guarantee
ordered access causing failures in device drivers that do not use the mandatory
memory barriers. readl & writel versions contain necessary memory barriers for
this.

commit 79f64dbf

: "ARM: 6273/1: Add barriers to
the I/O accessors if ARM_DMA_MEM_BUFFERABLE" can be referred for more
information on this.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>