target/arm: use FIELD macro for CNTHCTL bit definitions

target/arm: use FIELD macro for CNTHCTL bit definitions

We prefer the FIELD macro over ad-hoc #defines for register bits;
switch CNTHCTL to that style before we add any more bits.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240301183219.2424889-4-peter.maydell@linaro.org