clocksource/drivers/imx-tpm: Move tpm_read_sched_clock() under CONFIG_ARM

When building this driver for an architecture other than ARCH=arm:

drivers/clocksource/timer-imx-tpm.c:78:20: error: unused function 'tpm_read_sched_clock' [-Werror,-Wunused-function]
static u64 notrace tpm_read_sched_clock(void)
^
1 error generated.

Move the function definition under the existing CONFIG_ARM section so
there is no more warning.

Fixes: 10720e120e2b ("clocksource/drivers/imx-tpm: Exclude sched clock for ARM64")
Signed-off-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Peng Fan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Daniel Lezcano <[e-mail]>

drivers/clocksource/timer-imx-tpm.c

···
 {
 	return tpm_read_counter();
 }
-#endif
 
 static u64 notrace tpm_read_sched_clock(void)
 {
 	return tpm_read_counter();
 }
+#endif
 
 static int tpm_set_next_event(unsigned long delta,
 				struct clock_event_device *evt)