target/arm: change arch timer registers access permission

Some generic arch timer registers are Config-RW in the EL0,
which means the EL0 exception level can have write permission
if it is appropriately configured.

When VM access registers, QEMU firstly checks whether they have RW
permission, then check whether it is appropriately configured.
If they are defined to read only in EL0, even though they have been
appropriately configured, they still do not have write permission.
So need to add the write permission according to ARMV8 spec when
define it.

Signed-off-by: Dongjiu Geng <gengdongjiu@huawei.com>
Message-id: 1552395177-12608-1-git-send-email-gengdongjiu@huawei.com
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>