[POWERPC] 83xx: Add the MPC8315E RDB dts

Add the dts for the MPC8315E Reference Development Board (RDB).

The board is a mini-ITX reference board with 128M DDR2, 8M flash,
32M NAND, USB, PCI, gigabit ethernet, SATA, and serial.

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