hw/ssi/xilinx_spips: Avoid AXI writes to the LQSPI linear memory

hw/ssi/xilinx_spips: Avoid AXI writes to the LQSPI linear memory

Lei Sun found while auditing the code that a CPU write would
trigger a NULL pointer dereference.

>From UG1085 datasheet [*] AXI writes in this region are ignored
and generates an AXI Slave Error (SLVERR).

Fix by implementing the write_with_attrs() handler.
Return MEMTX_ERROR when the region is accessed (this error maps
to an AXI slave error).

[*] https://www.xilinx.com/support/documentation/user_guides/ug1085-zynq-ultrascale-trm.pdf

Reported-by: Lei Sun <slei.casper@gmail.com>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Tested-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>