aarch64.risu: add ReservedValue encodings
aarch64.risu: add ReservedValue encodings

add encodings that result in ReservedValue() for the currently supported
instructions.

Signed-off-by: Claudio Fontana <5fe97b97070bc75ff3c7a6b130e2dcc62cbb9b58@linaro.org>

vivier/risu,stsquad/risu,stsquad/risu,vivier/risu,stsquad/risu