target/arm: Don't allow RES0 CNTHCTL_EL2 bits to be written

target/arm: Don't allow RES0 CNTHCTL_EL2 bits to be written

Don't allow the guest to write CNTHCTL_EL2 bits which don't exist.
This is not strictly architecturally required, but it is how we've
tended to implement registers more recently.

In particular, bits [19:18] are only present with FEAT_RME,
and bits [17:12] will only be present with FEAT_ECV.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240301183219.2424889-5-peter.maydell@linaro.org