target/arm: Fix reads of CNTFRQ_EL0 in linux-user mode

target/arm: Fix reads of CNTFRQ_EL0 in linux-user mode

In commit bd8e9ddf6f6 ("target/arm: Refactor default generic timer
frequency handling") we changed how we initialized the generic timer
frequency as reported in the CNTFRQ_EL0 register.  As part of that,
we chanegd the linux-user version of the CNTFRQ_EL0 sysreg from
having a constant value set at compile time through the .resetvalue
field to having a reset value which we compute in a .resetfn.

This accidentally broke the reading of CNTFRQ_EL0 in linux-user mode,
because the cpreg is marked as ARM_CP_CONST, which means we translate
it as a read of the compile-time constant value in the .resetvalue
field.  This is now zero, so userspace sees a 0 frequency value.

Fix the bug by dropping the ARM_CP_CONST marking.  This will cause us
to translate the read as a load of the value from the CPU state
struct cp15.c14_cntfrq field, which is where the real frequency value
now lives.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3159
Fixes: bd8e9ddf6f6 ("target/arm: Refactor default generic timer frequency handling")
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: 20251013161040.216819-1-peter.maydell@linaro.org