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>