ARM: mxs_defconfig: Enable initramfs support

This makes it possible to automatically boot-test this defconfig with
kernelci.org.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/configs/mxs_defconfig

@@ -12,6 +12,7 @@ CONFIG_IKCONFIG_PROC=y
 # CONFIG_IPC_NS is not set
 # CONFIG_PID_NS is not set
 # CONFIG_NET_NS is not set
+CONFIG_BLK_DEV_INITRD=y
 CONFIG_PERF_EVENTS=y
 # CONFIG_COMPAT_BRK is not set
 CONFIG_MODULES=y