clocksource/drivers/imx-sysctr: Mark two variable with __ro_after_init

The variables 'sys_ctr_base' and 'cmpcr' are not be updated after
init, so mark them as __ro_after_init.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211201125030.2307746-1-peng.fan@oss.nxp.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>