xilinx_spips: allow mmio execution

This allows to execute from the lqspi area.

When the request_ptr is called the device loads 1024bytes from the SPI device.
Then this code can be executed by the guest.

Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>