imx6: add back perf monitor related config symbol
Commit 0543eb111081 ("imx6: 5.4: add missing kernel perf monitor
symbol") has added missing kernel config symbol FSL_IMX8_DDR_PMU which
is exposed only when building perf, so add it back.
Fixes: 8d9b36270b1f ("imx6: refresh kernel config")
Signed-off-by: Petr Štetiar <[email protected]>
Petr Štetiar