aarch64.risu: add comment on ld/st rn != rt constraints
aarch64.risu: add comment on ld/st rn != rt constraints

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

hw-claudio/risu-aarch64,hw-claudio/risu-aarch64