hw/ssi: xilinx_spips: Remove DMA related dead codes from zynqmp_spips

hw/ssi: xilinx_spips: Remove DMA related dead codes from zynqmp_spips

Now that the Xilinx CSU DMA model is implemented, the existing
DMA related dead codes in the ZynqMP QSPI are useless and should
be removed. The maximum register number is also updated to only
include the QSPI registers.

Signed-off-by: Xuzhou Cheng <[email protected]>
Signed-off-by: Bin Meng <[email protected]>
Reviewed-by: Edgar E. Iglesias <[email protected]>
Message-id: 20210303135254[email protected]
Signed-off-by: Peter Maydell <[email protected]>