layerscape: convert to squashfs rootfs for QSPI NOR boot

There had been an issue in Layerscape QSPI driver for very long
time, which made squashfs,jffs2 rootfs not work on QSPI NOR.
And the ubifs had been used as a workaround.
Now the issue has been fixed. So convert to use squashfs,jffs2
rootfs on QSPI NOR for Layerscape boards (LS1012ARDB/LS1046ARDB/
LS1088ARDB), and update u-boot bootargs for booting.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>