imx6: 5.4: add missing kernel perf monitor symbol

Add missing kernel config symbol probably some dependency uncovered by
my local perf related and enabled kernel config options.

 ARM PMU framework (ARM_PMU) [Y/n/?] y
 Freescale i.MX8 DDR perf monitor (FSL_IMX8_DDR_PMU) [N/m/y/?] (NEW)

Signed-off-by: Petr Štetiar <[email protected]>

target/linux/imx6/config-5.4

@@ -249,6 +249,7 @@ CONFIG_FIX_EARLYCON_MEM=y
 CONFIG_FSL_GUTS=y
 # CONFIG_FSL_QDMA is not set
 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