clocksource/drivers/imx-sysctr: Drop use global variables

Clean up code to not use global variables and introduce sysctr_private
structure to prepare the support for i.MX95.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240205-imx-sysctr-v4-2-ca5a6e1552e7@nxp.com