powerpc/fsl: fix "Failed to mount /dev: No such device" errors

Yocto (Built by Poky 7.0) 1.2 root filesystems fail to boot,
at least over nfs, with:

Failed to mount /dev: No such device

Configuring DEVTMPFS fixes it.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>