hw/ssi/xilinx_spi: Make device endianness configurable

hw/ssi/xilinx_spi: Make device endianness configurable

Replace the DEVICE_NATIVE_ENDIAN MemoryRegionOps by a pair of
DEVICE_LITTLE_ENDIAN / DEVICE_BIG_ENDIAN.

Add the "endianness" property to select the device endianness.
This property is unspecified by default, and machines need to
set it explicitly.

Set the proper endianness on the single machine using the
device.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250213122217.62654-7-philmd@linaro.org>