xilinx_spi: Switch to iomem functions and support little endian.

This patch changes the out_(be)(8|16|32) and in_(be)(8|16|32) calls to 32 bits ioread/iowrite.

The read and write function are attached to the internal struct as callbacks, callback
is selected depending on endianess.

This will also build on platforms not supporting the in/out calls for instance x86.

Acked-by: Grant Likely <grant.likely@secretlab.ca>
Tested-by: John Linn <John.Linn@xilinx.com>
Signed-off-by: Richard Röjfors <richard.rojfors@mocean-labs.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>