[POWERPC] 83xx: add the mpc837[789]_rdb dts files

Add the dts files for the MPC838xE Reference Development Board (RDB).

The board is a mini-ITX reference board with 256M DDR2, 8M flash,
32M NAND, USB, PCI, gigabit ethernet, SATA, and serial.

the difference among the three files is the 8377 has two, the 8378
none, and the 8379 has four sata controllers.

partially based on the 8379 mds device trees.

Signed-off-by: Joe D'Abbraccio <ljd015@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>