dmaengine: at_xdmac: fix usage of read, write wrappers

This driver uses read_relaxed and writel_relaxed to read, write to IO
memory. the config defines COMPILE_TEST so gets compiled on different archs.
This causes issue as few archs like x86 etc don't define it.
So use readl/writel which is defined in all archs

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>