powerpc/mpc8308rdb: support for MPC8308RDB board from Freescale

This patch adds support for MPC8308RDB development board from
Freescale.
Supported devices:
 DUART
 Dual Ethernet
 NOR and NAND flashes
 I2C
 USB in peripheral mode

PCIE support is broken by the commit 3da34aae ("powerpc/fsl: Support
unique MSI addresses per PCIe Root Complex"). Works after revert.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>