imx6: switch to 5.4 kernel

Seems to be working fine, it's 99.9% upstream stuff anyway. Lets start
wider userbase testing.

Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/imx6/Makefile

@@ -14,7 +14,7 @@ CPU_TYPE:=cortex-a9
 CPU_SUBTYPE:=neon
 MAINTAINER:=Luka Perkov <luka@openwrt.org>
 
-KERNEL_PATCHVER:=4.19
+KERNEL_PATCHVER:=5.4
 KERNEL_TESTING_PATCHVER:=5.4
 
 include $(INCLUDE_DIR)/target.mk