dm355 evm support

Initial U-Boot support for the DaVinci DM355 EVM.  This is a board
from Spectrum Digital.  Board docs include schematic and firmware
for its microcontroller:

  http://c6000.spectrumdigital.com/evmdm355/revd/

Most of the DM355 chip is fully documented by TI, the most notable
exception being the MPEG/JPEG coprocessor (programmable using codecs
available at no cost from TI), which is omitted from its DM335 sibling:

  http://focus.ti.com/docs/prod/folders/print/tms320dm355.html



This version can boot from the on-board DM9000 Ethernet chip, after
being loaded (from NAND, MMC/SD, or UART).  In the near future, NAND
and USB support could be added ... NAND support is being held back
until the support for the 4-bit ECC hardware is ready.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>