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]>

target/linux/imx6/config-5.4

@@ -202,6 +202,7 @@ CONFIG_FIXED_PHY=y
 CONFIG_FIX_EARLYCON_MEM=y
 CONFIG_FSL_GUTS=y
 CONFIG_FS_ENCRYPTION=y
+# CONFIG_FSL_IMX8_DDR_PMU is not set
 CONFIG_FS_IOMAP=y
 CONFIG_FS_MBCACHE=y
 CONFIG_FS_POSIX_ACL=y