hw/intc/arm_gicv3_its: Provide read accessor for translation_ops

hw/intc/arm_gicv3_its: Provide read accessor for translation_ops

The MemoryRegionOps gicv3_its_translation_ops currently provides only
a .write_with_attrs function, because the only register in this
region is the write-only GITS_TRANSLATER.  However, if you don't
provide a read function and the guest tries reading from this memory
region, QEMU will crash because
memory_region_read_with_attrs_accessor() calls a NULL pointer.

Add a read function which always returns 0, to cover both bogus
attempts to read GITS_TRANSLATER and also reads from the rest of the
region, which is documented to be reserved, RES0.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220122182444.724087-11-peter.maydell@linaro.org