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

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 <nathan@kernel.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20220303184212.2356245-1-nathan@kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>