Fixed multiple write by writing a DWORD over PCIe

[DmytroLevit]

Hi Heiko,

I've tried to use the module on my Kintex US board for configuring the SPI core, but run into a problem that the DWORD write transaction over PCIe is translated to 4 AXI transactions in FPGA.

This merge request fixes the problem by casting the memory mapped register space as array of integers.

[heikoengel]

Hi,
thanks a lot for this PR! I also noticed the multiple writes but it wasn't a problem for my use cases so far. The memoryview solution looks great! Will merge after I had a chance to give it a try in hardware myself, however that might not be in Dec anymore.