target/arm: HCR_EL2.RW should be RAO/WI if EL1 doesn't support AArch32

target/arm: HCR_EL2.RW should be RAO/WI if EL1 doesn't support AArch32

When EL1 doesn't support AArch32, the HCR_EL2.RW bit is supposed to
be RAO/WI. Enforce the RAO/WI behaviour.

Note that we handle "reset value should honour RES1 bits" in the same
way that SCR_EL3 does, via a reset function.

We do already have some CPU types which don't implement AArch32
above EL0, so this is technically a bug; it doesn't seem worth
backporting to stable because no sensible guest code will be
deliberately attempting to set the RW bit to a value corresponding
to an unimplemented execution state and then checking that we
did the right thing.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>