powerpc/fsl: Enable CONFIG_DEVTMPFS_MOUNT so /dev can be mounted correctly

When using recent udev, the /dev node mount requires CONFIG_DEVTMPFS_MOUNT
is enabled in Kernel. The patch enables the option in defconfig of Freescale
QorIQ targets.

Changed defconfig list:
   arch/powerpc/configs/85xx/p1023rds_defconfig
   arch/powerpc/configs/corenet32_smp_defconfig
   arch/powerpc/configs/corenet64_smp_defconfig
   arch/powerpc/configs/mpc85xx_smp_defconfig
   arch/powerpc/configs/mpc85xx_defconfig
   arch/powerpc/configs/mpc83xx_defconfig

Signed-off-by: Zhenhua Luo <zhenhua.luo@freescale.com>
[scottwood@freescale.com: added mpc83xx and non-smp mpc85xx]
Signed-off-by: Scott Wood <scottwood@freescale.com>