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

counter_width and timer_base will not be updated after init, so mark
as __ro_after_init.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20211214120737.1611955-4-peng.fan@oss.nxp.com


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