mv_xor: use {readl, writel}_relaxed instead of __raw_{readl, writel}

In order to support big-endian execution, the mv_xor driver is changed
to use the readl_relaxed() and writel_relaxed() accessors that
properly convert from the CPU endianess to the device endianess (which
in the case of Marvell XOR hardware is always little-endian).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Dan Williams <djbw@fb.com>