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

drivers/clocksource/timer-imx-sysctr.c

@@ -20,8 +20,8 @@
 
 #define SYS_CTR_CLK_DIV		0x3
 
-static void __iomem *sys_ctr_base;
-static u32 cmpcr;
+static void __iomem *sys_ctr_base __ro_after_init;
+static u32 cmpcr __ro_after_init;
 
 static void sysctr_timer_enable(bool enable)
 {